Add a generic post-prereq phase

This generic extra phase is handy for fixing up things like python
modules after pip is installed or other distro specific requirements.

Change-Id: I4a68b830cd98c6f93191dac1edd7de2a0381feaa
Add rhel6 as an allowed distro

Add RHEL6 based distributions as a valid install target

Change-Id: I89fb68d18a0d2079493be93b230162679123881d
Merge "Add rhel6 as an allowed distro"
Merge "Add a generic post-prereq phase"