Add quota support for LBaaS extension

As pointed out by the tempest tests, we also need to tell
the quota driver to support the LBaaS specific quota attributes.

This should fix
tempest.api.network.admin.test_quotas.QuotasTest.test_lbaas_quotas
Merge pull request #129 from dirkmueller/neutron_lbaas

Add quota support for LBaaS extension