DomainQuotaDriver implementation

This patch implements DomainQuotaDriver. This driver allows the control
of quotas at the domain level.
Merge branch 'domain_quota_driver_merged' of https://github.com/tellesnobrega/nova into domain_quota_driver_merged

Conflicts:
	nova/db/api.py
	nova/db/sqlalchemy/api.py
	nova/db/sqlalchemy/models.py
	nova/quota.py
	nova/tests/db/test_db_api.py
	nova/tests/db/test_migrations.py
	nova/tests/test_quota.py