Accept forbidden traits in the processing of extra_specs, with the
format of:
trait:CUSTOM_MAGIC=forbidden
This will be transformed into required=!CUSTOM_MAGIC when the traits
are assembled into a request to GET /allocation_candidates.
Implements blueprint forbidden-traits-in-nova
Change-Id: I31e609aef47d2fea03f279e4bfdd30f072d062b4