sshdriver: reduce loglevel when running commands

This avoids potential SSH log messages from ending up in the command output,
especially when using stderr_merge.

labgrid/driver/sshdriver.py

@@ -233,7 +233,7 @@ def _run(self, cmd, codec="utf-8", decodeerrors="strict", timeout=None):
         if not self._check_keepalive():
             raise ExecutionError("Keepalive no longer running")
 
-        complete_cmd = ["ssh", "-x", *self.ssh_prefix,
+        complete_cmd = ["ssh", "-x", "-o", "LogLevel=QUIET", *self.ssh_prefix,
                         "-p", str(self.networkservice.port), "-l", self.networkservice.username,
                         self.networkservice.address
                         ]