Add subcommand parser support in CLI

[Ravi Sankar Penta]

Current solum CLI workflow does not smoothly guide the new user.
For example, take the basic use case for creating a plan:

(a) $ solum help
...
usage: solum [-h] {component,app,assembly,pipeline,languagepack}
...

(b) $ solum app help
Available actions:
 create Create an application.
...

(c) solum app create help or --help
usage: solum [-h] [--os-username OS_USERNAME] [--os-password OS_PASSWORD]
             [--os-tenant-name OS_TENANT_NAME] [--os-auth-url OS_AUTH_URL]
             [--os-auth-token OS_AUTH_TOKEN] [--solum-url SOLUM_URL]
             [--solum-api-version SOLUM_API_VERSION]
             {component,app,assembly,pipeline,languagepack} action plan_file
...
Steps (a) and (b) are awesome but in step (c), we got a generic lengthy usage info and as we add new concepts it will be hard to understand easily. User is already in the 'app create' context and would like to know mandatory and optional parameters for application creation. More user friendly help would be:
usage: Application Creation
Params:
  plan_file - Application template file in YAML format...
Optional-Params:
  ...

By adding subcommand parsers, we can provide user friendly messages specific to app/assembly/etc.