RHEL6 core support

 - Allow RHEL6 as a valid target for installation

 - Qpid package name is different in RHEL6 to Fedora

 - Add some post-prereq actions:
  * pre-install dbus to avoid the service not having ever been started
  * a workaround for a hgtools/setuptools bug
  * install pycrypto and lxml as pip dependencies to ensure overrding
    of local versions
  * fix-up a dependency issue with prettytable by pre-installing

 - Split out the RHEL6 rpms into a separate list.  Various changes
   here to the base Fedora versions to not specify RHEL6 packages that
   are too old or conflict

 - The RHEL6 version of pip doesn't have support for mirrors, so skip
   asking for them.

 - Separate F18 polkit config and ensure it's written out for RHEL6,
   or you get authentication errors with libvirt

 - RHEL6 has no nodejs in main packages or in EPEL.  The easiest way
   is to just install the upstream binary version which works fine for
   the very minimal usage by lesscss.

 - By default RHEL6 qpidd ships with authentication on, which causes
   Kerberos errors; override this

Change-Id: I89aa507ae15df49383a32a1c4b4767acbf30bdf5