Fix some nitpicky stuff about docker groups

ping @aanand

1. Grammar
2. Commands not listed in global command list, which made discovery
difficult.
3. No YAML tags defined for some fields in group.yml.  Also no
volumes_from.
4. COMMAND => SUBCOMMAND
5. Padding in subcommand list was not enough for "containers"

Docker-DCO-1.1-Signed-off-by: Nathan LeClaire <nathan.leclaire@docker.com> (github: nathanleclaire)