knife_ssh plugin

This plugin adds a knife_ssh function as well as completion for it, to allow
connecting via ssh to servers managed with Chef.

To use it, add knife_ssh to the plugins array in your zshrc file:

plugins=(... knife_ssh)

The plugin creates a cache of the Chef node list via knife, and stores it
in $HOME/.knife_comp~, when first triggering knife_ssh completion.

Requirements: knife has to be installed.