Reservation with No Hosts Available returns 500

Extends manager errors with oshostplugin error: NotEnoughHostsAvailable
Exception thrown when there is not enought hosts available
for the physical host reservation.

Exception returns in API response as:
HTTP 500 No hosts available

Closes-Bug: #1298919
Change-Id: I2277a16b07f28c9a97d7bb3ff199ae46e35fce9b
Merge "Reservation with No Hosts Available returns 500"