Terraform Authentication (gophercloud#81)

This commit adds initial support for Terraform authentication.

The primary reason for this feature is to decouple the Terraform
Swift Remote State backend support from the Terraform OpenStack
Provider, thus reducing Terraform core's dependency of the OpenStack
Provider.

However, this package can also be used in any other situation where
a user wants to make an OpenStack-based Terraform provider/tool.

go.mod

@@ -3,5 +3,6 @@ module github.com/gophercloud/utils
 require (
 	github.com/gophercloud/gophercloud v0.0.0-20190212181753-892256c46858
 	github.com/hashicorp/go-uuid v1.0.1
+	github.com/mitchellh/go-homedir v1.1.0
 	gopkg.in/yaml.v2 v2.2.2
 )

go.sum

@@ -2,6 +2,8 @@ github.com/gophercloud/gophercloud v0.0.0-20190212181753-892256c46858 h1:QBK4YvY
 github.com/gophercloud/gophercloud v0.0.0-20190212181753-892256c46858/go.mod h1:vxM41WHh5uqHVBMZHzuwNOHh8XEoIEcSTewFxm1c5g8=
 github.com/hashicorp/go-uuid v1.0.1 h1:fv1ep09latC32wFoVwnqcnKJGnMSdBanPczbHAYm1BE=
 github.com/hashicorp/go-uuid v1.0.1/go.mod h1:6SBZvOh/SIDV7/2o3Jml5SYk/TvGqwFJ/bN7x4byOro=
+github.com/mitchellh/go-homedir v1.1.0 h1:lukF9ziXFxDFPkA1vsr5zpc1XuPDn/wFntq5mG+4E0Y=
+github.com/mitchellh/go-homedir v1.1.0/go.mod h1:SfyaCUpYCn1Vlf4IUYiD9fPX4A5wJrkLzIz1N1q0pr0=
 golang.org/x/crypto v0.0.0-20190211182817-74369b46fc67 h1:ng3VDlRp5/DHpSWl02R4rM9I+8M2rhmsuLwAMmkLQWE=
 golang.org/x/crypto v0.0.0-20190211182817-74369b46fc67/go.mod h1:6SG95UA2DQfeDnfUPMdvaQW0Q7yPrPDi9nlGo2tz2b4=
 golang.org/x/sys v0.0.0-20190209173611-3b5209105503 h1:5SvYFrOM3W8Mexn9/oA44Ji7vhXAZQ9hiP+1Q/DMrWg=