automation: execute powershell when connecting

For some reason, the ability to execute PS scripts appears to
come online after the ability to execute regular command scripts.
This is creating race conditions when connecting to instances
resulting in our wait_for_winrm() returning before PS is available
leading to an exception being thrown in other code.

Let's change the client connection code to execute a minimal
PS script so we can try to trap the exception in wait_for_winrm().

contrib/automation/hgautomation/winrm.py

@@ -37,7 +37,7 b' def wait_for_winrm(host, username, passw'
         try:
             client = Client(host, username=username, password=password,
                             ssl=ssl, connection_timeout=5)
-            client.execute_cmd('echo "hello world"')
+            client.execute_ps("Write-Host 'Hello, World!'")
             return client
         except requests.exceptions.ConnectionError:
             if time.time() >= end_time: