Refactored datastores to use common chown function

Refactored the mongdb and couchbase managers to use the common change_owner
function defined in trove/guestagent/common/operating_system.py instead
of rolling their own cmds to do the same thing.

Change-Id: Ia7cf3bacf32b50ebf857d5b43ed756e4b09596e9
Closes-bug: 1314803
Merge "Refactored datastores to use common chown function"