pdk's 2.1 ruby and gemset doesn't match the puppet-agent's

Initially reported in https://github.com/puppetlabs/pdk/issues/305:

The ruby bundled with pdk seems to not have readline support compiled in.

That specific dependency is pulled in through augeas (at https://github.com/puppetlabs/puppet-agent/blob/8d15e2a6a6ab8c7ba228271bf6b2f2b1ed914ffa/configs/components/augeas.rb#L26 and https://github.com/puppetlabs/puppet-agent/blob/8d15e2a6a6ab8c7ba228271bf6b2f2b1ed914ffa/configs/components/augeas.rb#L50).

Investigating this points to a number of components missing in the pdk that are deployed in the agent:

• augeas
• ruby-augeas
• ruby-stomp
• ruby-shadow
• libxml2
• libxslt
• ruby-selinux

There are other rubygems missing, but those are pure ruby libraries that'll get pulled in through the gem caching going on in pdk-module-template.