engine: Explicit MAC independent of pool size

Explicit MAC was changed by mistake to be dependent on the MAC pool
size, changing back so that if an explicit MAC address is specified the
pool is not checked if more MACs are available or not.

Change-Id: I073a6fafee23211f6a66a03767a6913b5b564650
Bug-Url: https://bugzilla.redhat.com/1039103
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>