Add ability to override a managed by description

[stack72]

Fixes: #93

This introduces the work to allow a resource to override a "Managed
by Terraform" description

[jen20]

This build is failing because of

pulumi-gcp/.travis.yml

Line 21 in cc6c26e

- openssl aes-256-cbc -K $encrypted_289ace6c29ed_key -iv $encrypted_289ace6c29ed_iv -in gcp-credentials.json.enc -out gcp-credentials.json -d

- in a fork, the secret is unavailable. We need to modify the build script here to test whether we're in a fork or not before attempting, and just skip anything requiring credentials if so.

[joeduffy]

This is awesome, LGTM 👍, thanks @stack72.