The neutron.tests.tools was recently made public and along with it the
WarningsFixture. However the WarningsFixture as-is doesn't support
filtering warnings from other modules. This patch address that by
accepting an constructor arg to add other module regular expressions to
filter. It also moves WarningsFixture to the fixture module where it
seems to make the most sense.
For a sample consumption patch see:
https://review.openstack.org/#/c/651370/
Change-Id: I08c713916aef407ba108ed2c67c3e99892f5883a
b4ecdc9945