906554: Add ui_repoid_vars line to yum based on the variables which are in the baseurl

Based on research done adding this value to the yum stanza has no negative impacts. This
was tested on RHEL6.4 and Fedora 17.

This patch assumes that yum variables look like

$variable_name

and not

some$variable_nameembedded

Although the latter can be supported.
Merge pull request #503 from candlepin/bkearney/906554

906554: Add ui_repoid_vars line to yum based on the variables which are ...