Rename gerrit-gertty.yaml to googlesource-gertty.yaml

The example configuration for gerrit-review.googlesource.com is also
applicable to other Gerrit instances running at googlesource.com, for
example:

 android-review.googlesource.com
 gwt-review.googlesource.com

Make the example generic, rather than specific to gerrit-review.

Change-Id: I761db4228aa23e92e57751f5ecbdf721c992065d