Update tox.ini to support RHEL 6.x.

In order to support running unit tests on RHEL 6.x we need to patch
eventlet with contrib/redhat-eventlet.patch. We already
have support for this in the tools/install_venv_common.py but we
need to make a couple changes to allow tox to consume this:

1) Sync in the latest intall_venv_common.py from oslo. This changes
 patch to use the -N option (ignore already applied patches) and
 makes it safe to call the patching function more than once.

2) Add a new patch_tox_venv.py script in tools.

3) Update tox.ini to call patch_tox_venv.py before it runs tests and
 coverage.

NOTE: This will hopefully go away eventually once this patch lands:

 https://bitbucket.org/eventlet/eventlet/issue/89/add-a-timeout-argument-to-subprocesspopen#comment-3342969

Change-Id: I3e2b07c3f718e4aede5c5f231ff0cdb7721ec885