Use guestagent conf set up by devstack

Devstack now sets up the trove-guestagent.conf in /etc/trove, so
we can (and should) just use that instead of copying the sample
guestagent conf over.

Change-Id: Ieab67ac29adef9929795cc5ffa1bc829bd07c422
Closes-bug: 1336090
Merge "Use guestagent conf set up by devstack"