Basic check for $DEST permissions

Several guides suggest using data directories under your homedir,
rather than the default /opt area.  This is fine, but on RHEL6 and
similar distros homedirs are very restrictive 0700 permissions which
doesn't allow things like httpd to pass through to serve up files.

Even though stack.sh is taking over the host, changing permissions
automatically is not a nice idea.  So we just warn when it looks like
this is happening.

Change-Id: I9cd70e7fe90638a2a5c3b8fd94756afacac7c7be
Add 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
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:
  * 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