Avoid installing python-nose for RHEL6.

 * python-nose on RHEL isn't new enough to support Tempest.

Change-Id: I98107bf664a12f6252f74b4511e3228d511ff855
Merge "Avoid installing python-nose for RHEL6."
Install EPEL for rhel6 if not detected

Simple check to see if EPEL repo is enabled for RHEL6 and, if not,
automated install of the repo RPM.

Also adds an additional sanity check which checking for pip.  In the
prior case of EPEL being disabled, a RHEL6 host may not have access to
python-pip.  Although this shouldn't happen, its a good sanity check.

Change-Id: I4e8a4dda9475b75d071534d4eef469198502a048
Ensure Open vSwitch can be installed on RHEL6.

 * RHEL6 does not include Open vSwitch in the default repos, but it
   is available via the RDO repo.  This patch automatically
   configures the RDO repo for RHEL6.
 * The openvswitch package is now listed as an rpm dependency to
   ensure it can be cached by a pre-built CI environment.

Change-Id: I8f93f53039cca4ff29bcb91720be7bb047b3ed8a
Force-install nose via pip for RHEL.

 * It's not possible to prevent python-nose from being installed
   since so many os packages dependent on it, so this patch forces
   pip installation of nose after os package installation.

Change-Id: I756d1ca5eda127dcdd2efb047c8e14dd344f710d