Removing the headroom calculation from db layer

It's odd that this is happening in the DB API because it's dealing
with instance quotas but maybe I'm not doing anything with instance
quotas, maybe I'm doing things with security group or fixed IP quotas.

This patch moves the headroom calculation from db layer to the
calling side.

Change-Id: I8379471eea4ea7b114c2a235224f6fe61f8f981f
Closes-bug: #1369696
Merge "Removing the headroom calculation from db layer"