never use ssh multiplexer in tunnels

`ssh -f` is [broken in OpenSSH](https://bugzilla.mindrot.org/show_bug.cgi?id=1948)
when a ControlMaster multiplexed connection is used.

`-S none` disables the multiplexed connection.

closes #4717
Merge pull request #4720 from minrk/ssh-no-mux

never use ssh multiplexer in tunnels

`ssh -f` is [broken in OpenSSH](https://bugzilla.mindrot.org/show_bug.cgi?id=1948) when a ControlMaster multiplexed connection is used.

`-S none` disables the multiplexed connection.

closes #4717