Minor useability improvement for arcctl

It would be nice if arcctl and sub-commands showed help information when no arguments are specified instead of having to use -h, eg in all the cases below the help info is shown instead of an error message:

$ arcctl 
usage: arcctl [-h] [-c CONFIG] [-d {CRITICAL,ERROR,WARNING,INFO,DEBUG}]
              COMPONENT ...
arcctl: error: too few arguments

$ arcctl help
usage: arcctl [-h] [-c CONFIG] [-d {CRITICAL,ERROR,WARNING,INFO,DEBUG}]
              COMPONENT ...
arcctl: error: argument COMPONENT: invalid choice: 'help' (choose from 'rte', 'job', 'service', 'deploy', 'accounting', 'config', 'test-ca', 'cache')

$ arcctl rte
usage: arcctl rte [-h] ACTION ...
arcctl rte: error: too few arguments