add firewall driver if we use quantum security group

fixes bug 1179820

we must set firewall driver if we want to use quantum security group because
quantum will disable security group if we do not change the default firewall
driver. Currently devstack will not change the default firewall driver
if we just running quantum server on a node which will cause nova unable to
security group information.

Change-Id: Ie274325decbf252630a237ed3d6ee3136eb259fe
specify session name on perform rejoin stack

rejoin-stack.sh would not specify screen session name which will lead
unstack.sh unable to kill the screen session started by rejoin-stack.sh
after hypervisor rebooted. So specify the session name if
rejoin-stack.sh need to start a new sreen session.

fixes bug 1182806

Change-Id: I68635887aa82fa63f3667e0f090ea2836eec7b68
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
Merge "specify session name on perform rejoin stack"
Merge "add firewall driver if we use quantum security group"
Merge "Install EPEL for rhel6 if not detected"