Updated the resourceTFEPolicySetUpdate function

[hcrhall]

Description

This PR contains a change to the resourceTFEPolicySetUpdate function within the tfe_policy_set resource that detects changes to the vcs_repo attributes. The issue is covered in more detail in the following:

Policy vcs_repo can't be updated #140

Testing plan

1. Create a main.tf:

provider "tfe" {
  version  = "0.18.0"
}

resource "tfe_policy_set" "test" {
  name          = "example_tfe_policy_set"
  description   = "example policy set"
  organization  = "my_org_name"

  vcs_repo {
    identifier         = "github_account/repo_name"
    branch             = "master"
    ingress_submodules = false
    oauth_token_id     = "token_id"
  }
}

2. Monitor a run and let provisioning occur
3. Update main.tf:

provider "tfe" {
  version  = "0.18.0"
}

resource "tfe_policy_set" "test" {
  name          = "example_tfe_policy_set"
  description   = "example policy set"
  organization  = "my_org_name"

  vcs_repo {
    identifier         = "github_account/repo_name"
    branch             = "my_other_branch_name" # <= updated property 
    ingress_submodules = false
    oauth_token_id     = "token_id"
  }
}

4. Monitor the run and check Policy Set in Terraform UI and confirm that branch name did change in alignment with the configuration change.

Output from acceptance tests

PASS tfe.TestAccTFEPolicySetParameter_basic (2.26s)
PASS tfe.TestAccTFEPolicySetParameter_update (3.42s)
PASS tfe.TestAccTFEPolicySetParameter_import (2.28s)
PASS tfe.TestAccTFEPolicySet_basic (2.38s)
PASS tfe.TestAccTFEPolicySet_update (4.08s)
PASS tfe.TestAccTFEPolicySet_updateWorkspaceIDs (5.31s)
PASS tfe.TestAccTFEPolicySet_updateEmpty (3.18s)
PASS tfe.TestAccTFEPolicySet_updatePopulated (4.40s)
PASS tfe.TestAccTFEPolicySet_updatePopulatedWorkspaceIDs (4.51s)
PASS tfe.TestAccTFEPolicySet_updateToGlobal (3.69s)
PASS tfe.TestAccTFEPolicySet_updateWorkspaceIDsToGlobal (3.75s)
PASS tfe.TestAccTFEPolicySet_updateToWorkspace (3.86s)
PASS tfe.TestAccTFEPolicySet_updateGlobalToWorkspaceIDs (3.72s)
PASS tfe.TestAccTFEPolicySet_vcs (4.02s)
PASS tfe.TestAccTFEPolicySet_updateToVcs (6.17s)
PASS tfe.TestAccTFEPolicySetImport (2.40s)
...

[lafentres]

would it be possible to avoid introducing another env var here? there are getting unwieldy. i'd love to find an alternative, if there is one, but it is pretty low priority.

the tfe_workspace tests also have a vcs repo and test going from master to GITHUB_WORKSPACE_BRANCH. is that something that would be possible to do here as well, to avoid the need for another env var?

[hcrhall]

@lafentres I've made a few changes to the vcs related tests so as to move away from our reliance on env vars. Take a look and let me know what you think.

[lafentres]

This looks good to me! I tested migrating from the old version to this new version, updating various VCS repo fields, and removing the VCS repo. Everything worked as expected.