Adds support for configuration of DNF repo mirrors for CentOS and EPEL
repositories, as well as custom repositories.

Adds support for DNF automatic, which is a replacement for yum-cron.

Configuration is backwards compatible, falling back to the equivalent
yum variables when DNF variables have not been overridden.

Change-Id: I8bef5e9c8e1c77c25d6077ff690da8f2cde6a643
Story: 2006574
Task: 38922