Remove Stop-Computer from PostReboot

to prevent shutting down too quickly after invoking sysprep, before it
completes.
this is a complementary change to bosh-psmodules use the /shutdown flag
in sysprep.exe for vsphere (bosh-psmodules repo bb07093d)

[#174128738](https://www.pivotaltracker.com/story/show/174128738)
 Update the stemcell creation process to ensure UTC timezone

Co-authored-by: Sam Coward <scoward@pivotal.io>
Co-authored-by: Anand Gaitonde <agaitonde@pivotal.io>
Co-authored-by: Sam Coward <scoward@pivotal.io>
Co-authored-by: Anand Gaitonde <agaitonde@pivotal.io>
Co-authored-by: Sam Coward <scoward@pivotal.io>
Co-authored-by: Anand Gaitonde <agaitonde@pivotal.io>

stemcell-automation/AutomationHelpers.Tests.ps1

@@ -61,23 +61,16 @@ Describe "PostReboot" {
         $postRebootCalls.IndexOf("CleanUpVM") | Should -BeLessThan $postRebootCalls.IndexOf("SysprepVM")
     }
 
-    It "syspreps right before shutting the VM down" {
+    It "syspreps as the last command" {
         PostReboot -Organization "org" -Owner "owner" -SkipRandomPassword:$false
         Assert-MockCalled -CommandName SysprepVM
         Assert-MockCalled -CommandName SysprepVM -ParameterFilter {
             $Organization -eq "org" -and
                     $Owner -eq "owner" -and
                     $SkipRandomPassword -eq $false
         }
-        $postRebootCalls.IndexOf("SysprepVM") | Should -BeLessThan $postRebootCalls.IndexOf("Stop-Computer")
-    }
-
-    It "powers off VM last" {
-        PostReboot
-
-        Assert-MockCalled -CommandName Stop-Computer
         $lastIndex = $postRebootCalls.Count - 1
-        $postRebootCalls.IndexOf("Stop-Computer") | Should -Be $lastIndex
+        $postRebootCalls.IndexOf("SysprepVM") | Should -Be $lastIndex
     }
 }
 

stemcell-automation/AutomationHelpers.ps1

@@ -37,8 +37,6 @@ function PostReboot
     InstallCFCell
     CleanUpVM
     SysprepVM -Organization $Organization -Owner $Owner -SkipRandomPassword $SkipRandomPassword
-    Write-Log "Completed Sysprep. About to poweroff VM..."
-    Stop-Computer
 }
 
 function CopyPSModules