Add ssh option to not use shared connections

I am using persistent SSH connections.

Part of my .ssh/config:
```
ControlMaster auto
ControlPath  /Users/udondan/.ssh/tmp/%h_%p_%r
ControlPersist 3600
```

For some reason, this prevents the exec() callback from being executed. The file downloads/uploads fine, just the cb is not run. 

Adding the ssh option **-o "ControlMaster no"** solves this issue. It's a pity, as shared ssh connections are meant to prevent re-connecting. The same ssh session would be used for multiple up- and downloads from/to the same host.