Clean nova.compute.api.API:_check_num_instances_quota

Simplification to a better understanding of total_allowed value
Originally:
total_allowed = used + headroom [resource]
with used = quotas [resource] - headroom [resource]
So total_allowed = quotas [resource]

Change-Id: Ie4b1f6cb8f0fd80610bdf3fb10ea0263281f56c0
Merge "Clean nova.compute.api.API:_check_num_instances_quota"