Moved quota headroom calculations into quota_reserve

Moved headroom calculations into quota_reserve and modified headroom
calculations to take into account -ve quota limits (unlimited) on
cores and ram.

Updated test cases to generate OverQuota exceptions with new headroom
parameter.

Added new tests for 'unlimited' quotas to test_quota.py in
 QuotaIntegrationTestCase and DbQuotaDriverTestCase classes.

Add Exception kwargs checking for OverQuota and QuotaError exceptions.

Change-Id: I855b0cad495182c5c4ee42b2acf5eedd198557b5
Closes-Bug: #1224453 min_count ignored for instance create