run_trigger: retry when a "locked" error is returned

[bendrucker]

Description

This PR adds retries for run trigger creation. Concurrently creating run triggers on a workspace results in an error—now those errors are considered retryable for 1 minute.

Closes #169

Testing plan

1. Create many run triggers for a given workspace
2. Attach them all in a single apply

I've added a new test that is able to reproduce this condition. When the test is run without the new retries, it fails with lock errors. In my initial test, 5/10 resources failed:

make testacc TESTARGS='-run TestAccTFERunTrigger_many'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run TestAccTFERunTrigger_many -timeout 15m
?   	github.com/terraform-providers/terraform-provider-tfe	[no test files]
=== RUN   TestAccTFERunTrigger_many
    TestAccTFERunTrigger_many: testing.go:569: Step 0 error: errors during apply:

        Error: Error creating run trigger on workspace ws-NNTPzRVLDRoHe6U9 with sourceable ws-KvgXdmjw462ZM2hq: Invalid Attribute

        Run Trigger creation locked by another user in your organization. Please try again

          on /var/folders/7p/z_kw6fc9693fly126tljpml40000gn/T/tf-test655254769/main.tf line 19:
          (source code not available)

   # 4 more errors truncated for brevity
--- FAIL: TestAccTFERunTrigger_many (5.48s)
FAIL
FAIL	github.com/terraform-providers/terraform-provider-tfe/tfe	5.771s
?   	github.com/terraform-providers/terraform-provider-tfe/version	[no test files]
FAIL
make: *** [testacc] Error 1

Output from acceptance tests

Please run the full suite of acceptance tests locally and include the output here.

$ make testacc TESTARGS='-run TestAccTFERunTrigger'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run TestAccTFERunTrigger -timeout 15m
?   	github.com/terraform-providers/terraform-provider-tfe	[no test files]
=== RUN   TestAccTFERunTrigger_basic
--- PASS: TestAccTFERunTrigger_basic (5.32s)
=== RUN   TestAccTFERunTrigger_basicWorkspaceID
--- PASS: TestAccTFERunTrigger_basicWorkspaceID (5.11s)
=== RUN   TestAccTFERunTrigger_updateWorkspaceExternalIDToWorkspaceID
--- PASS: TestAccTFERunTrigger_updateWorkspaceExternalIDToWorkspaceID (7.29s)
=== RUN   TestAccTFERunTrigger_many
--- PASS: TestAccTFERunTrigger_many (8.70s)
=== RUN   TestAccTFERunTriggerImport
--- PASS: TestAccTFERunTriggerImport (4.45s)
PASS
ok  	github.com/terraform-providers/terraform-provider-tfe/tfe	31.236s
?   	github.com/terraform-providers/terraform-provider-tfe/version	[no test files]

[bendrucker]

Resolved the merge conflict that'd resulted here, and re-ran the acceptance tests (edited the original post). Also noticed I should have been specifying the timeout in the schema, fixed that.

We're holding off on adopting run triggers until we can add several reliably, would appreciate a review here!

[koikonom]

Hi @bendrucker, thanks for submitting this PR.

[koikonom]

LGTM