redhat_register snippet - fix enabling repos

Because of a bug in RHEL 6.4 [1], repositories specified in
`subscription_manager_repos` parameter didn't get enabled when using
redhat_register template snippet. The `yum-config-manager --enable
...` command only works when `/etc/yum.repos.d/redhat.repo` file
already exists. But `subscription-manager register ... --auto-attach`
command will not create the repo file. We need to run
`subscription-manager repos --list` to trigger creation of the repo
file. In RHEL 6.5 the bug [1] has been fixed.

This workaround is related to Foreman BZ "[RFE] Use
subscription-manager in kickstart template, not rhnreg_ks" [2].

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1008016
[2] https://bugzilla.redhat.com/show_bug.cgi?id=1039910