Reserved service name “default” for single-instance services

[shivaraj-bh]

The other thing we can do here is to support a special service name, "default". Handy for cases where you run exactly 1 service, so both default namespace and the process name can drop it. Instead of db-init, it would just be postgres-init.

EDIT: Could happen in separate PR.

Originally posted by @srid in #258 (comment)