Add support for mirrorlist yumrepo parameter.

Added class parameters $enable_mirrorlist and $mirrorlisturl in order to
support changing the YUM repo mirrorlist URL similar to how this module
allows changing the baseurl URL.  This commit also changes the default
to use mirrorlist when appropriate, causing the configuration to be as
similar to the OS-supplied repo files.
Remove urlbit variable and restore $releasever.

This simplified things and brings us closer to the OS-supplied repo
files.
Merge branch 'feature/mirrorlist' of https://github.com/razorsedge/repo_centos into razorsedge-feature/mirrorlist
Merge branch 'razorsedge-feature/mirrorlist'