Fix devstack plugin custom config opt setting

Commit https://github.com/openstack/manila/commit/1a2ec3da introduced new
approach to manila configuration file setting and it won't consider custom
opts for DEFAULT group by default behaviour, only when its name added to
"MANILA_CONFIGURE_GROUPS".
We should make it set always, because DEFAULT group is used always.

Change-Id: I377d46148b8da3c26f26a51a42ae749715e396aa
Closes-Bug: #1401793