Support release repos

The nightly composes have different layout than the 1.2 release on CDN. It is a pity because otherwise openshift.sh could happily be used for installing OpenShift Enterprise 1.2.

The proposed patch compares CONF_RHEL_REPO and CONF_REPOS_BASE to see if they point to the same location (sans os/ subdirectory of CONF_RHEL_REPO). It then uses release style of baseurls for the individual OpenShift repositories.

For clean merge, https://github.com/openshift/openshift-extras/pull/38 is needed.