bootstrap: Wait for SSH daemon to become reachable

In some situations after restarting the SSH daemon by
“prepare-node-join” it's not immediately reachable. Like with the master
and node daemons, waiting for it to become reachable instead of failing
hard can help in such situations.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>