mgr/smb: add create_only arg for handler apply function

mgr/smb: add create_only arg for handler apply function

Add a create_only argument to the handler class apply function. This
flag is used to prevent modification of existing resources. This flag
will be use by 'cluster create' and 'share create' commands to make
them true to their names and not sneaky modify-or-create commands.

Signed-off-by: John Mulligan <jmulligan@redhat.com>