ml2/type_gre: Adds missing clear_db to test_type_gre.py

GreTypeTest.setUp() in test_type_gre.py should have
self.addCleanup(db.clear_db). This patch adds it.

Change-Id: I1f2ac5d2cae47812591fc5b50099053a4ad9ba85
Closes-Bug: #1261593
Merge "ml2/type_gre: Adds missing clear_db to test_type_gre.py"