CLI shows help without args

[rexwangcc]

Purpose

• https://github.com/broadinstitute/cromwell-tools/issues/97

Changes

• Some repo-level clean-ups.
• Fix the issue with CLI so it can show the help message when no commands/args provided.

Review Instructions

When run cromwell-tools, before:

TypeError: getattr(): attribute name must be string

now:

error: No commands/arguments provided!
usage: cromwell-tools [-h] [-V]
                      {submit,wait,status,abort,release_hold,metadata,query,health,task_runtime}
                      ...

positional arguments:
  {submit,wait,status,abort,release_hold,metadata,query,health,task_runtime}
                        sub-command help
    submit              submit help
    wait                wait help
    status              status help
    abort               abort help
    release_hold        release_hold help
    metadata            metadata help
    query               query help
    health              health help
    task_runtime        task_runtime help

optional arguments:
  -h, --help            show this help message and exit
  -V, --version         show program's version number and exit