Better argument validation for ZkTool.

Make `ZkTool` verify that the combination of arguments is valid and specifies one and only one action for `ZkTool` to perform. Print an error message and exit if not. Also, listing coordinates is now done with `ZkTool --list` instead of requiring an unused coordinate option with `ZkTool --operation LIST --coordinate 1.some.bogus.coordinate`.

If this pull request goes through, I'll update the wiki documentation in https://confluence.skunk-works.no/confluence/display/eng/Cloudname+Codelab and elsewhere to match the usage updates.