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 "Move enablement of rhel6-optional repo earlier"