glusterfs and glusterfs_native had a distinct
set of options to specify ssh credentials
(glusterfs_server_password vs glusterfs_native_server_password
and glusterfs_path_to_private_key vs glusterfs_native_path_to_private_key).
There is no reason to keep these separate; but worsening the situations
these options have been moved to layouts in an ad-hoc manner,
breaking certain driver/layout combos whereby the credential
option used by the driver is not provided by the chosen layout
and thus it was undefined.
Fix all the mess by defining glusterfs_server_password and
glusterfs_path_to_private_key in glusterfs.common, and
providing the native variants as deprecated aliases.
Change-Id: I48f8673858d2bff95e66bb7e72911e87030fdc0e
Closes-Bug: #1497212