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

@@ -1,82 +1,82 b''
 # winrm.py - Interact with Windows Remote Management (WinRM)
 #
 # Copyright 2019 Gregory Szorc <[e-mail]>
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 # no-check-code because Python 3 native.
 
 import logging
 import pprint
 import time
 
 from pypsrp.client import (
     Client,
 )
 from pypsrp.powershell import (
     PowerShell,
     PSInvocationState,
     RunspacePool,
 )
 import requests.exceptions
 
 
 logger = logging.getLogger(__name__)
 
 
 def wait_for_winrm(host, username, password, timeout=180, ssl=False):
     """Wait for the Windows Remoting (WinRM) service to become available.
 
     Returns a ``psrpclient.Client`` instance.
     """
 
     end_time = time.time() + timeout
 
     while True:
         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:
                 raise
 
             time.sleep(1)
 
 
 def format_object(o):
     if isinstance(o, str):
         return o
 
     try:
         o = str(o)
     except TypeError:
         o = pprint.pformat(o.extended_properties)
 
     return o
 
 
 def run_powershell(client, script):
     with RunspacePool(client.wsman) as pool:
         ps = PowerShell(pool)
         ps.add_script(script)
 
         ps.begin_invoke()
 
         while ps.state == PSInvocationState.RUNNING:
             ps.poll_invoke()
             for o in ps.output:
                 print(format_object(o))
 
             ps.output[:] = []
 
         ps.end_invoke()
 
         for o in ps.output:
             print(format_object(o))
 
         if ps.state == PSInvocationState.FAILED:
             raise Exception('PowerShell execution failed: %s' %
                             ' '.join(map(format_object, ps.streams.error)))