Ensure EPEL is configured before installing plugin

[alexjfisher]

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 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.

[alexjfisher]

Before this change...

Error: Execution of '/usr/bin/yum -d 0 -e 0 -y install python2-certbot-dns-rfc2136' returned 1: Error: Nothing to do

Full log:

Info: Applying configuration version '1585419866'
  Notice: /Stage[main]/Epel/File[/etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7]/ensure: defined content as '{md5}58fa8ae27c89f37b08429f04fd4a88cc'
  Notice: /Stage[main]/Letsencrypt::Config/File[/etc/letsencrypt]/ensure: created
  Notice: /Stage[main]/Letsencrypt::Config/File[/opt/puppetlabs/puppet/cache/letsencrypt]/ensure: created
  Notice: /Stage[main]/Letsencrypt::Config/Ini_setting[/etc/letsencrypt/cli.ini server https://acme-staging-v02.api.letsencrypt.org/directory]/ensure: created
  Notice: /Stage[main]/Letsencrypt::Config/Ini_setting[/etc/letsencrypt/cli.ini email letsregister@example.com]/ensure: created
  Notice: /Stage[main]/Letsencrypt/File[/usr/local/sbin/letsencrypt-domain-validation]/ensure: defined content as '{md5}aef2b04a89ed35fabcc3ce4b2ae60e99'
  Error: Execution of '/usr/bin/yum -d 0 -e 0 -y install python2-certbot-dns-rfc2136' returned 1: Error: Nothing to do
  Error: /Stage[main]/Letsencrypt::Plugin::Dns_rfc2136/Package[python2-certbot-dns-rfc2136]/ensure: change from 'purged' to 'present' failed: Execution of '/usr/bin/yum -d 0 -e 0 -y install python2-certbot-dns-rfc2136' returned 1: Error: Nothing to do
  Notice: /Stage[main]/Epel/Epel::Rpm_gpg_key[EPEL-7]/Exec[import-EPEL-7]/returns: executed successfully
  Notice: /Stage[main]/Epel/Yumrepo[epel-testing]/ensure: created
  Info: Yumrepo[epel-testing](provider=inifile): changing mode of /etc/yum.repos.d/epel-testing.repo from 600 to 644
  Notice: /Stage[main]/Epel/Yumrepo[epel-testing-debuginfo]/ensure: created
  Info: Yumrepo[epel-testing-debuginfo](provider=inifile): changing mode of /etc/yum.repos.d/epel-testing-debuginfo.repo from 600 to 644
  Notice: /Stage[main]/Epel/Yumrepo[epel-testing-source]/ensure: created
  Info: Yumrepo[epel-testing-source](provider=inifile): changing mode of /etc/yum.repos.d/epel-testing-source.repo from 600 to 644
  Notice: /Stage[main]/Epel/Yumrepo[epel]/ensure: created
  Info: Yumrepo[epel](provider=inifile): changing mode of /etc/yum.repos.d/epel.repo from 600 to 644
  Notice: /Stage[main]/Epel/Yumrepo[epel-debuginfo]/ensure: created
  Info: Yumrepo[epel-debuginfo](provider=inifile): changing mode of /etc/yum.repos.d/epel-debuginfo.repo from 600 to 644
  Notice: /Stage[main]/Epel/Yumrepo[epel-source]/ensure: created
  Info: Yumrepo[epel-source](provider=inifile): changing mode of /etc/yum.repos.d/epel-source.repo from 600 to 644
  Notice: /Stage[main]/Letsencrypt::Install/Package[letsencrypt]/ensure: created
  Info: Class[Letsencrypt::Install]: Scheduling refresh of Exec[initialize letsencrypt]
  Notice: /Stage[main]/Letsencrypt::Renew/File[letsencrypt-renewal-hooks-puppet]/ensure: created
  Notice: /Stage[main]/Letsencrypt/Exec[initialize letsencrypt]: Triggered 'refresh' from 1 event
  Notice: /Stage[main]/Letsencrypt::Plugin::Dns_rfc2136/File[/etc/letsencrypt/dns-rfc2136.ini]/ensure: defined content as '{md5}98119d5f0c43ddf05a4d12ab4d073130'
  Info: Class[Letsencrypt::Plugin::Dns_rfc2136]: Unscheduling all events on Class[Letsencrypt::Plugin::Dns_rfc2136]
  Info: Stage[main]: Unscheduling all events on Stage[main]