openshift.ks: Optionally register with RHN/RHSM

configure_rhn_channels, configure_rhsm_channels: Only register with RHN or RHSM if credentials are provided.
configure_rhsm_channels: Only attach to a pool if a pool id is provided.

validate_preflight: Allow the installation to proceed even if the rhn or rhsm installation method is being used without any credentials or a pool id provided.

These changes allow the user to register with RHN or RHSM before running the script and then use the rhn or rhsm installation method to have the installation script configure channels, priorities, and excludes and validate the configuration.

This commit fixes bug 1056906.