Update Windows Server 2019 docker image names

windows_2019_docker.json

@@ -170,7 +170,7 @@
     "autounattend": "./answer_files/2019_core/Autounattend.xml",
     "disk_size": "61440",
     "disk_type_id": "1",
-    "docker_images": "mcr.microsoft.com/nanoserver:sac2019 mcr.microsoft.com/windowsservercore:ltsc2019 mcr.microsoft.com/windows:ltsc2019",
+    "docker_images": "mcr.microsoft.com/windows/nanoserver:1809 mcr.microsoft.com/windows/servercore:ltsc2019",
     "docker_provider": "ee",
     "docker_version": "18-03-1-ee-3",
     "headless": "false",

windows_2019_docker_azure.json

@@ -96,7 +96,7 @@
     "autounattend": "./answer_files/2019/Autounattend.xml",
     "disk_size": "51200",
     "disk_type_id": "1",
-    "docker_images": "mcr.microsoft.com/nanoserver:sac2019 mcr.microsoft.com/windowsservercore:ltsc2019 mcr.microsoft.com/windows:ltsc2019",
+    "docker_images": "mcr.microsoft.com/windows/nanoserver:1809 mcr.microsoft.com/windows/servercore:ltsc2019",
     "docker_provider": "ee",
     "docker_version": "18-03-1-ee-3",
     "headless": "false",