Lazy load CONF.quota_driver.

nova.quota is imported before config options are parsed and it looks at
CONF.quota_driver on module-load.  This changes it so we look at
CONF.quota_driver post-module-load.

Fixes bug 1154371

Change-Id: Ia2e63a7aeab2234be683ff507e7856d46601a9a8
Merge "Lazy load CONF.quota_driver."