Add a pull request hook to run benchmark on every PR.

[tejal29]

It would be great to make sure a PR is not degrading the build time. This will also help us identify PRs which improve the performance especially when snapshotting the whole File System.
We have seen some bugs reported by users like #970.

The idea is to

1. Use https://developer.github.com/webhooks/ to write a service hosted on GCP which will be called on every new PR or changes to an existing PR.
2. Run Benchmark on every PR BENCHMARK=true for a set of real dockerfiles (not all)
3. The service, will grab the results from the current run, compare it with master and see if the PR caused a performance regression.

Changes:

1. Refactor code to identify if ./benchmark.sh is run inside a Travis Run.
   • reuse some of the logic in Run integration tests in Travis CI #979
2. Currently BENCHMARK=true uploads a file to gs://kaniko-project/benchmarks for every Dockerfile. Change this to publish just one file gs://kaniko-project/benchmarks/<pull_req_num>
3. Finally write the webservice which
   • pull the results for master stored at gs://kaniko-project/benchmarks/master
   • pull the results for the current PR stored at ``gs://kaniko-project/benchmarks/<pull_req_num>`
   • response if diff +ve or -ve time difference.