Cleanup backends after each test

Instance variables lingering around after a test completes can waste a
significant amount of memory during a long test run. This commit adds a
general helper method that can be used by add test classes to clean up
instance variables.

Change-Id: Ie7554f4c2e6e80540ea9f393f29b0eb15cacdf8c
Cleanup of instance attrs in core tests

This will cleanup the instance attributes added by setUps in
keystone.tests.core.

Change-Id: Iba820cbd33403c75291f04fd330bf343721e590b
Cleans up test data from limit tests

Change-Id: Ibd10e800b3108bfbb5bce076d3c1566cac3eb138
Cleanup fixture data added to test instances

Change-Id: I1c8f603dc520f2483630a902a4f93fd5f04b05b4
Cleanup keystoneclient tests

I remove redundant calls to load backends and initialize the database to
hopefully make these tests a little faster. I also added some cleanup
for instance attributes to help with memory consumption.

Change-Id: If2afec11266e0cd4cd132800259c4d4f84644600
Very minor cleanup to default_fixtures

The import wasn't being used so I removed it. Fixed a minor whitespace
issue.

Change-Id: I240250a45ab9e233964869e571b7ee84b03c7c74
Merge "Cleanup backends after each test"
Merge "Cleanup of instance attrs in core tests"
Merge "Cleans up test data from limit tests"
Merge "Cleanup fixture data added to test instances"
Merge "Cleanup keystoneclient tests"
Merge "Very minor cleanup to default_fixtures"