Enable Quota DB driver by default

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
Merge "Enable Quota DB driver by default" into milestone-proposed