Remove RUBYOPT var from Puppet runs

This is no longer needed per gds/puppet@0e23694
Use govuk_puppet for Puppet runs

It is now more important that we use our wrapper script instead of the
vanilla `puppet` binary because it handles:

- Ruby versioning through rbenv (gds/puppet@4995ab2).
- Nagios passive checks (gds/puppet@8a7e61e).

It already applies the arguments `agent --onetime --no-daemonize`.
Merge pull request #30 from alphagov/puppet_commands

Use govuk_puppet for Puppet runs