Change init type of BackwardCompatibilityTaskCreationTest

Since this test fails sporadically, added an after method to run after
each test that resets the DbFacade static reference to avoid it from
affecting other tests

This is the failure log from the tests:
Tests in error:
testConcreateCreateTaskBackwardsComaptibility(org.ovirt.engine.core.bll.
BackwardCompatibilityTaskCreationTest):
testConcreateCreateTaskBackwardsComaptibility(data[6])

Change-Id: Ic51dcdbe554844698d1897c5323b0a4290ba10c9
Signed-off-by: Tal Nisan <tnisan@redhat.com>