Utilize assertIsInstance

Using assertTrue and the 'isinstance' function to test
if an object is in an instance of some class is too python2.4.
Our unit testing framework supports assertIsInstance which was created
for these types of tests. Let's use assertIsInstance for these tests.

Fixes bug #1230028

Change-Id: Ia03c9f19a1c5e2aef3a6fe530515bd26339a7975
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"
Merge "Utilize assertIsInstance"