set valid versions for vagrant boxes

Author: crobert-1

deployments/ansible/molecule/config/windows.yml

@@ -10,7 +10,7 @@ driver:
 platforms:
   - name: "2016"
     box: StefanScherer/windows_2016
-    box_version: 2019.02.14
+    box_version: ${MOLECULE_VAGRANT_BOX_VERSION:-2019.02.14}
     box_url: ${MOLECULE_VAGRANT_BOX_URL}
     cpus: 2
     memory: 8192
@@ -30,7 +30,7 @@ platforms:
       - "winrm.retry_delay = 10"
   - name: "2019"
     box: gusztavvargadr/iis-windows-server
-    box_version: 2102.0.2312
+    box_version: ${MOLECULE_VAGRANT_BOX_VERSION:-2102.0.2312}
     box_url: ${MOLECULE_VAGRANT_BOX_URL}
     cpus: 2
     memory: 8192