Usergrid 2350

Regenerate App Credentials was failing with an HTTP Status of 400 and I was able to reproduce that failure with a new test (added in this pull request) called testResetAppCredentials().

For some reason ApplicationResources.generateKeys() was annotated with "@Produces(MediaType.TEXT_HTML)". When I changed that to "@Produces(MediaType.APPLICATION_JSON)" the problem went away.

Since I did some formatting of code, here's a link to the commit that has those two changes above:
https://github.com/apigee/usergrid-stack/commit/b32dfbdc6be80dcbec57de976cd33e2c5fc8adcb