Add support for multiple level CLI commands

E.g. "docker groups create" will attempt to call the function
CmdGroupsCreate

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Merge pull request #8128 from bfirsh/two-level-cli-commands

Add support for multiple level CLI commands
Remove the graphdb (links). Replace with code and json structure.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)