fixing the ssh test

Author: cderici

juju/machine.py

@@ -140,7 +140,7 @@ async def _scp(self, source, destination, scp_opts):
             raise JujuError("command failed: %s" % cmd)
 
     async def ssh(
-            self, command, user=None, proxy=False, ssh_opts=None):
+            self, command, user='ubuntu', proxy=False, ssh_opts=None):
         """Execute a command over SSH on this machine.
 
         :param str command: Command to execute

juju/unit.py

@@ -223,7 +223,7 @@ def set_meter_status(self):
         raise NotImplementedError()
 
     async def ssh(
-            self, command, user=None, proxy=False, ssh_opts=None):
+            self, command, user='ubuntu', proxy=False, ssh_opts=None):
         """Execute a command over SSH on this unit.
 
         :param str command: Command to execute