Add tests/common to .gitignore
Add tests/common to .gitignore

Running 'tox -e linters' generates files in 'tests/common' which
then triggers git to show them as changed files.

Change-Id: Ie3c992b4fa3ab2a460c3f9a14295280d086f1f27

os-cloud/openstack-ansible-galera_server,openstack/openstack-ansible-galera_server,os-cloud/openstack-ansible-galera_server,openstack/openstack-ansible-galera_server