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
Do not run selinux commands if it is disabled.

 * Execution of the setenforce command when selinux was not enabled
   was resulting in a non-zero exit status, which halted devstack.
 * Addresses bug 1175444

Change-Id: I7f6492dea7c52d153e755cda826d6fabd53f3771
Ensure Open vSwitch can be installed on RHEL.

 * RHEL6 does not include Open vSwitch in the default repo, but it
   is available via the RDO repo.  This patch automatically
   configures the RDO repo if required.

Change-Id: I8f93f53039cca4ff29bcb91720be7bb047b3ed8a
Install new nose for tempest on RHEL.

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

Change-Id: I98107bf664a12f6252f74b4511e3228d511ff855