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 happen.

Scroll down to the very bottom of the diff to see the meaningful changes in `deisctl` code. You can test by rebuilding it and varying the timeout value. With my DO instances I need at least 3.3 seconds' SSH timeout apparently.

Closes #2556.