Fallback to Quota Conf Driver if Quotas table is not defined

commit de15e0b9c5 enabled Quota DB driver default considering
production environments, but it breaks plugins without per-tenant
quota extension. In these plugin quotas tables is not loaded.

This commit fallbacks to ConfDriver if Quota model is not loaded by
checking neutron.db.quota_db which defines Quota model is imported.

Change-Id: Idaaaa9810598cfd3e5ce70020f498643b4819d16
Closes-Bug: #1236993
Use L3 api from vpn ipsec driver via service plugin

VPNaaS and ML2 plugin won't work, because ML2 plugin
supports service version of L3.
In this commit, we modify ipsec driver to use L3 plugin.
This is also backward compatible change, because if L2 plugin
supports L3 get_service_plugin API returns L2 plugin.

Fixes bug 1238846

Change-Id: I36e541bb2e3e1df2e01f73a74f3e9005af6c38b7
Note: Exsiting unit test covers this change
Merge "Fallback to Quota Conf Driver if Quotas table is not defined"
DB migration upgrade/downgrade not employed for brocade-plugin

Fixes bug: 1238921

Change-Id: I441d82a92588d7318a2f3b725643bc1b050a2c9c
Merge "DB migration upgrade/downgrade not employed for brocade-plugin"
Merge "Use L3 api from vpn ipsec driver via service plugin"