feat(deisctl): add -ssh-timeout command-line option

This change re-vendors CoreOS' `fleet` code to incorporate their `--ssh-timeout` option in coreos/fleet#1009, and makes similar changes to allow `deisctl --ssh-timeout=30 <command>`. Thanks to @lsnyder for making this happend.

Scroll down to the very bottom of the diff to see the meaningful changes in `deisctl` code.

Closes #2556.