Deprecate unused entries in cfg.py

backup_namespace and restore_namespace are now referenced on a
per-datastore basis. Having them in the common options is now no
longer useful. Deleted them from the common_options and marked them as
deprecated in the datastores.

Change-Id: I2d021d68c483b77f9426d78685a06fb0733ec5e9
Closes-Bug: #1351545
Merge "Deprecate unused entries in cfg.py"