Ensure EPEL is configured before installing plugin

I've noticed other PRs fail because puppet is trying to install the
dns_rfc2136 package before EPEL has been configured. The cleanest way of
fixing this seems to be to have the plugin class require letsencrypt.
There are only two resources in the letsencrypt::plugin::dns_rfc2136
and the file resource already required Class['letsencrypt']. Now the
package resource will too.

The base class already contains letsencrypt::install and that
includes epel and configures the correct ordering.