Consider outputting help for a specific context when command is empty

[filippsen]

Given the following command:

sns CONTEXT

Current output:

[ERROR] Unknown <CONTEXT> command ''

Suggested output:

Usage:
help
-h
    Output this help

version
-V
    Output the version of Simplenetes

sns CONTEXT COMMAND [OBJECT] [OPTIONS] [ARGUMENTS]

Cluster commands:
  cluster create <cluster name>
[...]

Host commands:
  host register <host>
[...]


Pod commands:
  pod ls [[pod[:version]][@host]]
[...]

Or just that particular help usage. Example:

sns host

Targetted output:

sns host COMMAND [OBJECT] [OPTIONS] [ARGUMENTS]

Host commands:
  host register <host>
[...]

Could also consider applying the same to invalid commands, but that is more understandable as it is (in comparison to the case of empty commands).
Invalid command:

sns CONTEXT invalid

Current output:

[ERROR] Unknown <CONTEXT> command 'invalid'