Update reset_db to call setup if _DB is None.
Update reset_db to call setup if _DB is None.

This resolves issues when running individual tests that require
a database. With this fix I can once again run individual test
modules that make use of the database. For example:

  ./run_tests.sh test_libvirt

Previously this would fail with database errors.

Fixes LP Bug #1032738.

Change-Id: Icce7ac9414f0e19eece44819a217634947de7f73

n0ano/gantt,n0ano/gantt