Add new resource tfe_agent_token

[jgrumboe]

Description

This PR adds a new tfe_agent_token resource that can be used to create agent tokens using the Agent Tokens API endpoint.

This PR depends on PR hashicorp/go-tfe#169 in the hashicorp/go-tfe repository, which added the files agent_token.go and associated agent_token_test.go to interact with the above API endpoint.

Closes #250.

Testing plan

The new tfe_agent_token resource can be tested with this command after following the setup instructions in TESTS.md:

TESTARGS="-run TestAccTFEAgentToken" make testacc

However, testing prior to the merging of hashicorp/go-tfe#169 and issuing a new release of go-tfe would require you to build go-tfe locally from https://github.com/jgrumboe/go-tfe/tree/jgrumboe-agent-token and then add a line like replace github.com/hashicorp/go-tfe => /path-to-local-repo/go-tfe to go.mod above the required stanza.

External links

• Agent Tokens API
• Required go-tfe PR

Output from acceptance tests

TESTARGS="-run TestAccTFEAgentToken" make testacc
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run TestAccTFEAgentToken -timeout 15m
?       github.com/hashicorp/terraform-provider-tfe     [no test files]
=== RUN   TestAccTFEAgentToken_basic
--- PASS: TestAccTFEAgentToken_basic (31.41s)
PASS
ok      github.com/hashicorp/terraform-provider-tfe/tfe 31.675s
?       github.com/hashicorp/terraform-provider-tfe/version     [no test files]

[chrisarcand]

This looks great! Just needs a few last minute bits:

[chrisarcand]

Build's failing because of #263