ftp, cifs: Make optional prefixes really optionals.

Excludes and remove were marker optional, but weren't in practice.
Provides defaults values so that they are really optional.

Change-Id: I6bd6f5183d888b0716bdf5690fc19bcd91bfc856
Reviewed-on: https://review.openstack.org/23864
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins