Closes-Bug: #1189671
Quota driver is now loaded in lazy mode, i.e. the driver is loaded
the first time the driver is accessed. This is to make unit tests
work. Some unit tests like extension test cases need to use Config
Quota driver (previous default) but QuotaEngine is initialized
when quota.py is imported. Thus the unit tests had no chance to
specify quota_driver.
Change-Id: I9e20961d5a6322361e3c0284b3c2a7ca86755c70