Allow to overwrite tags if testfile is used

This small change allows to overwrite the dddb_tag (and conddb_tag) from the test file database in the run_bandwidth_test_jobs script when testfiledb_key is used. Right now the dddb_tag argument in the YAML file is ignored if testfiledb_key is present.

This capability is needed for tests of new hlt2 lines, or more specifically their testing on simulation. See lhcb/Moore!3533 (merged) for more details.

Also see the corresponding issue #29.