Change Cassandra to Service Start vs Bin

starts cassandra with 'sudo service cassandra start' vs.
'sudo /usr/sbin/cassandra' to avoid having cassandra stopped if the
guestagent is stopped. see the bug details for more information.

Change-Id: Ifab557aaff5023f4b0fb4b91193785a0deb5ee2f
Closes-Bug: #1295313
Merge "Change Cassandra to Service Start vs Bin"