Allow some commands to run without full config validation #1973
Description
This has come up a couple times in the last week (most recently in #1972).
Commands like stop, kill, ps, logs, port, restart, rm, start only need to know the list of service names, not any part of the service definition.
Commands like build, pull only need to care about one or two fields from the service definition (build and dockerfile for build, image for pull).
If we could split our config validation into two phases (a top level service name validation, and a full validation) we could support that first group of commands.
The second group would be more work, and require a three phase validation.