Delete DB records instead of tables to speedup UT

Delete DB records instead of tables to speedup UT

Now that the schema is fixed for all of the plugins,
there isn't a need to delete and recreate the entire
schema for every unit test.

This patch clears the tables at the end of each test
instead of deleting them. This eliminated overhead seems
to save 10%+ execution time of the entire set of unit
tests.

Example of performance gain from tox -epy27 tests.unit.ml2:
Before: Ran 2495 tests in 284.186s
After: Ran 2495 tests in 223.299s

Change-Id: Ic5bcbb0cf941e0745890abc776d719e58bb42e35