Make sure to import Posh-Ohai module in PowerShell

While PowerShell version 3 will load the module automatically,
PowerShell version 2 requires you to be more explicit.

This was missing in from 239fa1e3c7aa. We discovered this in
interactive debugging session with Hounsou Dansou.

Change-Id: I88db408c00b8d880cec08a14086e605e99ff8799

satori/sysinfo/posh_ohai.py

@@ -67,7 +67,8 @@ def system_info(client, with_install=False):
         perform_install(client)
 
     if client.is_windows():
-        powershell_command = 'Get-ComputerConfiguration'
+        powershell_command = ('Import-Module -Name Posh-Ohai;'
+                              'Get-ComputerConfiguration')
         output = client.execute(powershell_command)
         unicode_output = "%s" % output
         load_clean_json = lambda output: json.loads(get_json(output))

satori/tests/test_sysinfo_posh_ohai.py

@@ -38,7 +38,8 @@ class TestSystemInfo(utils.TestCase):
     def test_system_info(self):
         self.client.execute.return_value = "{}"
         posh_ohai.system_info(self.client)
-        self.client.execute.assert_called_with("Get-ComputerConfiguration")
+        self.client.execute.assert_called_with("Import-Module -Name Posh-Ohai;"
+                                               "Get-ComputerConfiguration")
 
     def test_system_info_json(self):
         self.client.execute.return_value = '{"foo": 123}'