Misc doc fix + make clusters argument required

[UncleBarney]

1. Fix on command help doc.
2. Make clusters argument as required.

[opaugam]

A few remarks : all tools are defaulted to match '*' by design (especially log, grep and the like). You don't want to force the user to specify a regex when he or she wants to list the whole thing. What you want though is to add an extra confirmation switch to the ping tool, exactly the same way it's done for 'kill'. I suggest to rollback your changes and just mimic the logic from kill.py into ping.py.

[UncleBarney]

You are absolutely right, your plan makes more sense, I will update this PR.

[opaugam]

thanks !