need a pip-install test

The image-metadata-injection task added an optional config file, but the filename was added to setup.cfg and as a result, Glance installation failed because the file does not exist in the distribution. This was fixed by https://review.openstack.org/#/c/537453/ , but we need to add some kind of test to make sure this doesn't happen again, because this failure broke a lot of integration tests (e.g., legacy-requirements-integration-dsvm in the requirements gate):

 error: can't copy 'etc/glance-image-import.conf': doesn't exist or not a regular file
 ERROR: the main setup script run by this job failed - exit code: 1
     please look at the relevant log files to determine the root cause
 Running devstack worlddump.py
 Cleaning up host