Remove leading slash in layer tarball paths (Closes: #726)

[xanonid]

This PR removes leading slashes in the path of files in the layer.tar tarballs. This is necessary to be more compatible with docker and some security scanners.

Closes: #726

[tejal29]

@xanonid thank you for the PR and sorry for delay.
Currently some of the integration tests are failing.

--- FAIL: TestRun/test_Dockerfile_test_mv_add (3.44s)
        integration_test.go:224: diff = [
              {
                "Image1": "gcr.io/kaniko-test/docker-dockerfile_test_mv_add",
                "Image2": "gcr.io/kaniko-test/kaniko-dockerfile_test_mv_add",
                "DiffType": "File",
                "Diff": {
                  "Adds": [
                    {
                      "Name": "/tmp/tars",
                      "Size": 10813
                    },
                    {
                      "Name": "/tmp/tars/file",
                      "Size": 12
                    },
                    {
                      "Name": "/tmp/tars/file.bz2",
                      "Size": 196
                    },
                    {
                      "Name": "/tmp/tars/file.tar",
                      "Size": 10240
                    },
                    {
                      "Name": "/tmp/tars/file.tar.gz",
                      "Size": 199
                    },
                    {
                      "Name": "/tmp/tars/sys.tar.gz",
                      "Size": 166
                    }
                  ],
                  "Dels": null,
                  "Mods": null
                }
              },
              {
                "Image1": "gcr.io/kaniko-test/docker-dockerfile_test_mv_add",
                "Image2": "gcr.io/kaniko-test/kaniko-dockerfile_test_mv_add",
                "DiffType": "Metadata",
                "Diff": {
                  "Adds": [],
                  "Dels": []
                }
              }
            ]
        integration_test.go:227: []integration.diffOutput differ (-got, +want): {[]integration.diffOutput}[0].Diff.Adds:
            	-: []integration.fileDiff{{Name: "/tmp/tars", Size: 10813}, {Name: "/tmp/tars/file", Size: 12}, {Name: "/tmp/tars/file.bz2", Size: 196}, {Name: "/tmp/tars/file.tar", Size: 10240}, {Name: "/tmp/tars/file.tar.gz", Size: 199}, {Name: "/tmp/tars/sys.tar.gz", Size: 166}}
            	+: []integration.fileDiff(nil)

--- FAIL: TestRun/test_Dockerfile_test_run (10.21s)
        integration_test.go:224: diff = [
              {
                "Image1": "gcr.io/kaniko-test/docker-dockerfile_test_run",
                "Image2": "gcr.io/kaniko-test/kaniko-dockerfile_test_run",
                "DiffType": "File",
                "Diff": {
                  "Adds": [
                    {
                      "Name": "/etc/baz",
                      "Size": 4
                    }
                  ],
                  "Dels": null,
                  "Mods": null
                }
              },
              {
                "Image1": "gcr.io/kaniko-test/docker-dockerfile_test_run",
                "Image2": "gcr.io/kaniko-test/kaniko-dockerfile_test_run",
                "DiffType": "Metadata",
                "Diff": {
                  "Adds": [],
                  "Dels": []
                }
              }
            ]
        integration_test.go:227: []integration.diffOutput differ (-got, +want): {[]integration.diffOutput}[0].Diff.Adds:
            	-: []integration.fileDiff{{Name: "/etc/baz", Size: 4}}
            	+: []integration.fileDiff(nil)

Let me know if you need more help in figuring these failures.

[xanonid]

@tejal29 After rebasing and with my latest commit, which remove also the leadings slash from whiteout file paths, the integration test seems to run now fine. Could you, please, trigger kokoro again?

PS: To get it really running (local docker version 18.09.7), I needed to apply following change (the corresponding error message is mentioned in #368 ):

diff --git a/pkg/util/fs_util.go b/pkg/util/fs_util.go
index 4abfcd4..be9ef7b 100644
--- a/pkg/util/fs_util.go
+++ b/pkg/util/fs_util.go
@@ -99,6 +99,11 @@ func GetFSFromImage(root string, img v1.Image) ([]string, error) {
                        path := filepath.Join(root, filepath.Clean(hdr.Name))
                        base := filepath.Base(path)
                        dir := filepath.Dir(path)
+                       if base == ".wh..wh..opq" {
+                               // FIXME: skip to prevent error in next step
+                               continue
+                       }
+
                        if strings.HasPrefix(base, ".wh.") {
                                logrus.Debugf("Whiting out %s", path)
                                name := strings.TrimPrefix(base, ".wh."