Parameterise the version of unicornherder
Parameterise the version of unicornherder

This will allow divorcing puppet version from the change to
unicornherder, allowing it to be rolled out in a staged manner.

I have fixed the version in all environments currently to
[0.0.4](https://github.gds/gds/deployment/commit/d401f899f294972b05a4208b1ec05bda19d8deeb) and we can slowly roll this out, then
switch the default version from 0.0.4 to 0.0.8.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet