(maint) enable 'add_el_extras' for Beaker

Internally, we removed EPEL repos from our acceptance VM templates.
We found that, for our testing to be successful, we have a package
or two that comes from EPEL. This commit sets 'add_el_extras' to 'true'
so that Beaker can setup the EPEL repos on a SUT.
Merge pull request #2906 from rji/maint/stable/add_el_extras

(maint) enable 'add_el_extras' for Beaker