Make the space override responder-agnostic
https://fedorahosted.org/sssd/ticket/2397
In order to make the override_space option usable by other responders,
we need to move the override_space option to the generic responder
structure.
Reviewed-by: Pavel Březina <pbrezina@redhat.com>
Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>