Add server kill option

[sogorkis]

I found it necessary to sometimes kill all presto processes on all machines. I used custom scripts for that with pkill -9 presto command.

It would be useful to have kill command in presto-admin. For example:

presto-admin server kill -9
Are you sure you want to kill presto cluster nodes (y/n, default: n): y
Killing ...

[cawallin]

kill is a part of the launcher scripts, so we can easily add this (though I'm not sure we'd get to it this sprint). Thanks for the feedback!

[kbajda]

maybe "presto-admin server force-kill" ?