Make govuk_puppet re-exec self for root privs
Make govuk_puppet re-exec self for root privs

Rather than duplicating the commands for root and non-root usage. Switch
shell to bash to use `$BASH_SOURCE` (self). Remove `exec` from `puppet
agent` because we're going to add another command after it and the exec
isn't strictly necessary.

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