Override the default RHEL releasever

RHEL versions default releasever includes the minor versions or
6Server or 7Server. we need to override this to get epel-release
installed.
Don't think we want to leave this on the machine, now that I think about it.  Either rm the file as well at the end, 
or move the content of bootstrap-epel.repo content into bootstrap_epel method and use a Ruby variable in it.