Add zsh completion generator

Add completion generator script for zsh. Using native zsh completion functions
we can auto-complete things like instance/node names, job ids, nic/disk/backend
parameters etc.

The generated completion file can be copied to e.g.
/usr/share/zsh/functions/Completion/Unix/_ganeti under Debian, or any other
location under $fpath on other systems.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>