Adding client commands for Backup feature

- Added create, list and delete backup commands
- Updated the create instance command to include optional param 'BackupRef'
- Added 'backups' subcommand to 'instance' used to list all backups for that instance.

Change-Id: Icc32e7097858708d0ed0f4c309d4c86f5cbd8aa6
BP: https://blueprints.launchpad.net/reddwarf/+spec/consistent-snapshots