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>
Add basic host management with drivers
Make driver an option of "hosts create"
Remove redundant ToJSON method
Add support for setting options on a driver
Remove unused error from NewStore
Add support for choosing host with -H
Display error if url not passed to socket driver
Rename Driver.Name to DriverName for clarity
Add basic virtualbox driver

Creates a host that doesn't do anything.
Make socket driver options private
Store socket URL as property of driver
Fix valid host name regex
Print full command name in subcommand help text