Some params should be necessary when we use "barbican secret store" command

[Jeremy Liu]

When you store a secret without payload, and then try to retrieve that secret's payload, you will get
raise StorePluginNotAvailableOrMisconfigured(plugin_name)
StorePluginNotAvailableOrMisconfigured: The requested Store Plugin None is not currently available. This is probably a server misconfiguration.

Which is misleading exception and it needs to report proper error/info message, clearly telling user that the "payload is not found" or "No Payload present"

steps to reproduce the issue
1. barbican secret store
2. barbican secret get <Secret_href>
3. barbican secret get <Secret_href> --payload

Get the following trace
Traceback (most recent call last):
  File "/opt/stack/service/barbican-api-20160520T213237Z/venv/lib/python2.7/site-packages/barbican/api/controllers/__init__.py", line 103, in handler
    return fn(inst, *args, **kwargs)
  File "/opt/stack/service/barbican-api-20160520T213237Z/venv/lib/python2.7/site-packages/barbican/api/controllers/__init__.py", line 89, in enforcer
    return fn(inst, *args, **kwargs)
  File "/opt/stack/service/barbican-api-20160520T213237Z/venv/lib/python2.7/site-packages/barbican/api/controllers/secrets.py", line 184, in payload
    **kwargs
  File "/opt/stack/service/barbican-api-20160520T213237Z/venv/lib/python2.7/site-packages/barbican/api/controllers/secrets.py", line 162, in _on_get_secret_payload
    transport_key)
  File "/opt/stack/service/barbican-api-20160520T213237Z/venv/lib/python2.7/site-packages/barbican/plugin/resources.py", line 132, in get_secret
    secret_metadata.get('plugin_name'))
  File "/opt/stack/service/barbican-api-20160520T213237Z/venv/lib/python2.7/site-packages/barbican/plugin/interface/secret_store.py", line 489, in _check_plugins_configured
    return plugin_related_function(self, *args, **kwargs)
  File "/opt/stack/service/barbican-api-20160520T213237Z/venv/lib/python2.7/site-packages/barbican/plugin/interface/secret_store.py", line 557, in get_plugin_retrieve_delete
    raise StorePluginNotAvailableOrMisconfigured(plugin_name)
StorePluginNotAvailableOrMisconfigured: The requested Store Plugin None is not currently available. This is probably a server misconfiguration.

Actually, if we use "barbican secret store" command without providing a "payload", that doesn't make any sense.
So I'm going to make the "payload" param necessary when we use "barbican secret store" command.