Big Switch: Import DB module in unit test

Import the portbinding db module in the Big Switch base test case
so the table gets created before the test is run. Otherwise the
tests will fail when being run with testtools because the portbinding
db isn't normally imported until a function call from the Plugin.

Closes-Bug: #1331240
Change-Id: Icbd2248a0c013c142ccf9b302a1ffa15c5a4e5ae
Merge "Big Switch: Import DB module in unit test"