Drop "credentials" input and prep for 1.0 release (#279)

This drops the long-deprecated "credentials" input and pulls many of the
expensive integration tests into unit tests which assert the same
behavior. It also switches to using the JSON output from the "gcloud app
deploy" command instead of parsing the text output, since that has
proven to be unstable.

Closes #166
Closes #255
Closes #268
Closes #277 
Closes #278

Author: sethvargo