Add ability to run nailgun tests in parallel on ci

TEST_NAILGUN_DB=nailgun_gw0 nosetests nailgun/test

nosetests uses entry_points, so it requires to run command:

python setup.py develop

Change-Id: Ie5a24d970f73ef35b1c4e61bbc4b2a738d96b34f
Closes-Bug: #1282565
Merge "Add ability to run nailgun tests in parallel on ci"