Add management_project_id for Group and Project Clusters

[magic7s]

Adds management_project_id as an option to Group and Project Clusters.
Requires GitLab 12.10
Option Documentation: https://docs.gitlab.com/ee/api/group_clusters.html#add-existing-cluster-to-group

[magic7s]

Moving to draft until GitLab 12.10 is released and tests will pass.

[magic7s]

Tests passing on gitlab.com version GitLab Enterprise Edition 12.10.0-pre 462a6166989

$ TESTARGS="-run Cluster" make testacc
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test -v ./gitlab -run Cluster -timeout 40m
=== RUN   TestAccGitlabGroupCluster_basic
--- PASS: TestAccGitlabGroupCluster_basic (59.58s)
=== RUN   TestAccGitlabGroupCluster_import
--- PASS: TestAccGitlabGroupCluster_import (20.81s)
=== RUN   TestAccGitlabProjectCluster_basic
--- PASS: TestAccGitlabProjectCluster_basic (47.39s)
=== RUN   TestAccGitlabProjectCluster_import
--- PASS: TestAccGitlabProjectCluster_import (16.97s)
PASS
ok  	github.com/terraform-providers/terraform-provider-gitlab/gitlab	145.229s

[magic7s]

Ready for first review. Tests passing.

[magic7s]

@roidelapluie wondering if you could take a look at this. It's blocking another project. Thanks!

[magic7s]

@ringods could you please review this?

[ringods]

@magic7s if you could rebase this PR and squash some of your commits for a cleaner history, I'll merge immediately after that. You will also notice that the EE tests will be skipped instead of fail after the rebase, which is more clear as an input for merging. 😉

[ringods]

Failing test is related to #319.

[magic7s]

Thanks @ringods ! I thought PRs would squash on merge.

[magic7s]

Now we just need a new release when you are ready. =)

[ringods]

@magic7s Github as an option to squash on merge but I usually choose not to do that. It is your work, so I rather let you choose how much commits you leave in the PR and the commit comments.
Also, Github will not rebase automatically on merge. Given a number of PRs are open for quite a while, I am interested to see each set of changes on the tip of master to prevent unexpected behaviour.

[ringods]

Released in v2.9.0