Isolate tests from the environment variable http_proxy.
Isolate tests from the environment variable http_proxy.

This fixes bug 1078548.

This is done by adding a dependency on the fixtures library, which has
dedicated code for just this sort of thing, and using it in the base class.

Change-Id: I841fbf912b1a3ab43ca8c52c779d930aaf4a0e96

zancas/hacking,openstack-dev/hacking,emonty/oslo-hacking,zancas/hacking,hyakuhei/cleantox,emonty/oslo-hacking,hyakuhei/cleantox,openstack-dev/hacking