(maint) Allow epel inclusion to be disabled

Because epel 7 is not signed entirely, we have disabled gpgcheck on
those repos. That means that the epel module can't be included again or
instantiated in this module. This commit adds a parameter to the module
that allows epel inclusion to be toggled on or off.
Merge pull request #82 from haus/maint/master/allow_epel_to_be_not_included

(maint) Allow epel inclusion to be disabled