allow webapp service account id to be customised

As noted in #6, we were hard-coding the service account id used for the
webapp to "webapp-run". This meant it was impossible to deploy more than
one webapp in a project.

Form a better default from the "name" variable. For existing deployments
the service account id will be unchanged if the "name" variable is at
its default value.

Allow the service account id to be specified explicitly via the
optional "service_account_id" variable.

Closes #6