Modify distro message for Dev VM
Modify distro message for Dev VM

Warn if a version other than Precise is set. Don't nag if it's already set
to Precise, but will nag if we change the const in the future. We should aim
to phase out Lucid support.

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