Test RHEL repositories daily in Travis
Ticket moved to GitHub: "pulp/pulp_rpm/2248":https://github.com/pulp/pulp_rpm/issues/2248
Every now and then users report issues which are specific to RHEL content but currently we are testing only CentOS and Fedora. We also need to test certificates, see #6735.
Get a test cert for CDN and configure it in Travis, see https://pulp.plan.io/issues/6735#How-to-add-these-tests.
Add tests to sync and publish the latest RHEL6, RHEL7 and RHEL8 (BaseOS and AppStream) repos. Those tests should run daily.