service: lint and add missing argspec elements specs

roles/service/meta/argument_specs.yaml

@@ -13,6 +13,7 @@ argument_specs:
         type: list
         required: false
         default: []
+        elements: str
       service_container_http_port:
         description:
           - Port inside the container where http requests will be proxied to.
@@ -29,6 +30,7 @@ argument_specs:
         type: list
         required: false
         default: []
+        elements: str
       service_container_mounts:
         description: List of bind mounts or volumes to be mounted inside the service container(s).
         type: list
@@ -125,6 +127,7 @@ argument_specs:
             type: list
             required: false
             default: []
+            elements: str
           env:
             description: A dict of environment variables for the container
             type: dict