DOC: also bump suggested resource min in docs

[#173163022](https://www.pivotaltracker.com/story/show/173163022)

Add a test to verify optional configs are valid

- Verify ytt can process each optional config file
- Also verify that adding the optional config changes the output
(otherwise it isn't needed and should be removed)

[#172598032](https://www.pivotaltracker.com/story/show/172598032)

Co-authored-by: Eric Promislow <epromislow@suse.com>

The docker image smoke test still needs the diego_docker flag to be set

[#172251124](https://www.pivotaltracker.com/story/show/172251124)