Supply DEPLOY_ENV to paas-billing
Supply DEPLOY_ENV to paas-billing

We want `paas-billing`'s logs to say which PaaS environment it is
running in. This commit passes it the `DEPLOY_ENV` environment
variable that we already use for that purpose.

alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf