Support list of Red Hat channels and repos

When registering to RHN or RHSM a list of space-separated
channels or repositories may be provided. This allows control
over which packages are available to the system during build.
Note, the optional channel or repository is added by default
since diskimage-builder requires access to its packages. This
list is for additional channels or repos.

Change-Id: I9cf90d47ab1d09f5491f574132410438f9e294e3