Implement config support for drbd static minors

This patch adds support for allocating static minors.

Like for the LVM uuids, we add a new cache for the temporarily allocated
requests, and the users of the new methods must manually clear the
cache. If this doesn't happen, at worst we lose some minors.

Todos remaining:
  - implement duplicate check at configuration load, and at instance
    add/instance update
  - investigate automatically cleaning the cache on instance add/update
    if the minors/instance/node pairs match

Reviewed-by: imsnah