[Terraform] Add more OAuth2 credential options.

[emilymye]

• use golang.org/x/oauth2/google CredentialsFromJSON method to parse credentials file -
• Add access_token parameter to allow authorizing TF via a temporary non-renewable OAuth2 access token

---

[all]

[terraform]

Add OAuth2 access_token and client config as credential options.

[terraform-beta]

Add OAuth2 access_token and client config as credential options.

[ansible]

[inspec]

[modular-magician]

I am a robot that works on MagicModules PRs!

I built this PR into one or more PRs on other repositories, and when those are closed, this PR will also be merged and closed.
depends: hashicorp/terraform-provider-google-beta#330
depends: hashicorp/terraform-provider-google#2838

[modular-magician]

I am (still) a robot that works on MagicModules PRs!

I just wanted to let you know that your changes (as of commit 63fa64d) have been included in your existing downstream PRs.

[modular-magician]

I am (still) a robot that works on MagicModules PRs!

I just wanted to let you know that your changes (as of commit 2ddc5d1) have been included in your existing downstream PRs.

[rileykarson]

Seems generally sensible- I just had one question inline, and another about how we're testing this. Is it infeasible to test?

[emilymye]

@rileykarson added some acc tests for config. Not really sure if I like this test but it's config-specific enough to put in config_test.go?

[rileykarson]

That seems like the right place, and the test looks reasonable enough; can you add docs as well to https://www.terraform.io/docs/providers/google/provider_reference.html#configuration-reference?

[emilymye]

make testacc TEST=./google TESTARGS='--run="TestAccConfig"'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 TF_SCHEMA_PANIC_ON_ERROR=1 go test ./google -v --run="TestAccConfig" -timeout 120m -ldflags="-X=github.com/terraform-providers/terraform-provider-google/version.ProviderVersion=acc"
=== RUN TestAccConfigLoadValidate_credentials
--- PASS: TestAccConfigLoadValidate_credentials (0.69s)
=== RUN TestAccConfigLoadValidate_accessToken
--- PASS: TestAccConfigLoadValidate_accessToken (0.19s)
PASS
ok github.com/terraform-providers/terraform-provider-google/google 1.326s