Logging audit for guestagent/cassandra

Adjust logging to conform to logging standards.
Cleaned up some messages that were unclear and
changed many messages from LOG.info to LOG.debug.

Fixed a pattern of exception handlers using LOG.error
to log the exception (instead of LOG.exception)

Change-Id: I828fbb38b6bf6eb36c349d83475ca46c28779225
Partial-Bug: #1324206
Merge "Logging audit for guestagent/cassandra"