Update install_venv_common from oslo-incubator

Just some python3 compat work.

Change-Id: I835765d45a861d0bcf9b348f0649c6c9ef5a1992
Copy the RHEL6 eventlet workaround from Oslo

Tests in oslo-incubator now need the horrendous hack to workaround an
issue with eventlet on RHEL6. We've moved the patch_tox_venv tool
and redhat-eventlet.patch into oslo-incubator, so add them to
openstack-common.conf.

Change-Id: I1acfec282b64d7582144b155119bedb1741db2f7
Merge "Update install_venv_common from oslo-incubator"
Merge "Copy the RHEL6 eventlet workaround from Oslo"