Add CRD shortnames

[edavidj]

What this PR does / why we need it?:

• Adds the proposed shortnames for appsody CRD: app and apps
• Improves ease of use when retrieving resources

Does this PR introduce a user-facing change?

• [x] User guide
• [x] CHANGELOG.md

Which issue(s) this PR fixes:

Fixes #198

Notes:

• I'm not certain whether these shortNames have been finalized, if not feel free to close or hold off on the review.
• There was no reference to names in user-guide.md, currently I've opted to not add this but if we want this documented I can add in a short section.
• Note that operator-sdk open-api generated files were changed from relative paths to the module name in many places.

[leochr]

@edavidj Thank you. Shortname changes look good. I agree it'll be good to add some documentation.

I am not sure about the cause for path changes in OpenAPI generated files. Please ensure you are using operator-sdk version v0.12.0 with go v1.13+