Removes the dependence with aptitude

Removes the dependence with aptitude by replacing
the call of:
  aptitude purge -y ~npackage by apt_get purge -y package*

Change-Id: I08875ffad9dc6293047827666f02453a355b16ea
Closes-Bug: 1281410
Move enablement of rhel6-optional repo earlier

Change 4f1fee6eae300a3384900df06ebc857e95854eb0 added the RHEL6
optional repo in fixup_stuff.sh, but it turns out that doesn't get run
until after the package prerequisites phase.  Move this into stack.sh
with the RDO repo setup.

Change-Id: Iae0df85fa94c6c1b6f497dd29fda90d03b903a41
Merge "Removes the dependence with aptitude"
Merge "Move enablement of rhel6-optional repo earlier"
Merge branch 'master' of https://github.com/openstack-dev/devstack