Fix tmp DB path calculation for test_migrations.py

The method used by glance/tests/unit/test_migrations.py for
calculating tmp DB paths for sqlite connections works only when the
test tmp DB is in the current directory. When the test tmp DB is in
another directory (specified by an absolute path), the path calculated
is wrong. Consequently the test resetting function thinks the DB has
been reset (because the path calculated does not exist) but the actual
test fails because the db in the actual path has not been cleaned up.

This commit corrects the path calculation logic to handle absolute
paths as well.

Fixes bug 1263639

Change-Id: If747da25f40d92cf10ac5ac8addeb768f90e38cd
Merge "Fix tmp DB path calculation for test_migrations.py"