Launch / Exec - better defaults

[davidslater]

This makes two small changes:

1. Instead of "launch" defaulting to a no-op and shutting down without --interactive or --jupyter provided, it now defaults to --interactive:

armory launch pytorch

2. Instead of "exec" erroring out when no command line args are given, it defaults to a no-op.

armory exec pytorch --

Exec will still error if the -- delimiter is not present.