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.

Change-Id: I3e2b07c3f718e4aede5c5f231ff0cdb7721ec885

tox.ini

@@ -8,7 +8,9 @@ setenv = VIRTUAL_ENV={envdir}
          LC_ALL=C
 deps = -r{toxinidir}/tools/pip-requires
        -r{toxinidir}/tools/test-requires
-commands = python setup.py testr --slowest --testr-args='{posargs}'
+commands =
+  python tools/patch_tox_venv.py
+  python setup.py testr --slowest --testr-args='{posargs}'
 
 [tox:jenkins]
 sitepackages = True
@@ -30,7 +32,9 @@ commands = python tools/flakes.py nova
 [testenv:cover]
 # Also do not run test_coverage_ext tests while gathering coverage as those
 # tests conflict with coverage.
+setenv = VIRTUAL_ENV={envdir}
 commands =
+  python tools/patch_tox_venv.py
   python setup.py testr --coverage \
     --testr-args='^(?!.*test.*coverage).*$'