Update/Upgrade trove-guestagent code

Trove needs mechanism for updating(during one release cycle)/upgrading(from one release to another) guest code after migration from one release to another, taking into account possible guest RPC API changes.

Warning: Note, user should perform backup before update/upgrade. There's no way to downgrade.

Since guestagent is an application which could be delivered into many kind ways, we need to extend mgmt API calls stack.

Task #1: Versionize guest RPC API
      Define RPC API version per Trove release.

Task #2: Update/Upgrade
      Define pluggable mechanism for guest update/upgrade.
      Requires: update/upgrade strategy
      Request parameters:
            DBInstance ID
                        Type: UUID
            StrategyName.
                        Type: String.
                        Description: Update/Upgrade strategy.
                        Possible values: from_source, from_package
      Flow: trove-manage guest-update 5ff34c3d-689f-46f0-904a-9c1cc4d54fcb --strategy from_source
                If strategy available - execute update/upgrade, restart instance, send finish_update call

      Required configuration:
            Available strategy names;
            Strategy implementations;
            Version of current guest RPC API;
            Strategy related parameters (from_source: code repository, from_package: package URL).

Task #3: finish_update/upgrade call
      Perform database check. (mysql ping, nodetool statusthrift, etc)
      Requires: check process implementation. ( suc, err = util.execute_with_timeout('bla-bla') )
      Flow: cast finish_update(): suc, err = util.execute_with_timeout('mysql ping', run_as_root=True, shell=True)