Fix accessing to undefined command & argument vars

Although in the command.conf file almost all arguments have been defined with $powershell_*$,
but in the example.conf file the arguments and commands are accessed with vars.winrm_*, which do not exist.

icinga2/command.conf

@@ -1,4 +1,4 @@
-object CheckCommand "winrm" {
+object CheckCommand "powershell" {
   command = [ PluginDir + "/check_by_powershell" ]
 
   arguments = {

icinga2/example.conf

@@ -2,10 +2,10 @@ object Host "windowshost" {
   import "generic-host"
 
   address = "192.168.172.217"
-  vars.winrm_port = 5985
-  vars.winrm_user = "windowsuser"
-  vars.winrm_password = "secret!pw"
-  // vars.winrm_insecure = true
+  vars.powershell_port = 5985
+  vars.powershell_user = "windowsuser"
+  vars.powershell_password = "secret!pw"
+  // vars.powershell_insecure = true
 
   vars.server_type = "windows"
 }
@@ -14,13 +14,13 @@ template Service "powershell-template-service" {
    import "generic-service"
 
    vars.original_check_command = check_command
-   check_command = "winrm"
+   check_command = "powershell"
 
-   vars.by_winrm_command = {{ get_check_command(service.vars.original_check_command).command }}
-   vars.by_winrm_arguments = {{ get_check_command(service.vars.original_check_command).arguments }}
+   vars.by_powershell_command = {{ get_check_command(service.vars.original_check_command).command }}
+   vars.by_powershell_arguments = {{ get_check_command(service.vars.original_check_command).arguments }}
 }
 
-apply Service "winrm_check_ntp" {
+apply Service "powershell_check_ntp" {
   check_command = "powershell-checkTime-sync"
 
   import "powershell-template-service"