Adds help for subcommands

Adds help subcommand which takes other subcommands as param.
Adds -h/--help option for subcommands.

This contains integration testing code for tuskar CLI.
Following tests are implemented:
o bare help command
o help command with bad parameter
o help command with -h / --help parameter
o set of tests for "help <command>", "<command> -h / --help" for following
commands:
  - flavor-create
  - flavor-delete
  - flavor-list
  - flavor-show
  - flavor-update
  - rack-create
  - rack-delete
  - rack-list
  - rack-show
  - rack-update

Fixes bug in tests/test_shell.py - {} -> []

Fixes bug: 1213050

Change-Id: Ic554198f4fb4cdbaeefb9831c15f969301a7be87