engine: resources: Add another validation check

This simple check should prevent some silly mistakes and make the logic
easier for other parts of the code that won't have to worry about this
pattern.

test/shell/t2.yaml

@@ -21,8 +21,6 @@ resources:
     state: exists
   - name: file4
     path: "/tmp/mgmt/f4"
-    content: |
-      i am f4 and i should not be here
     state: absent
   - name: dir1
     path: "/tmp/mgmt/dir1/"

test/shell/t6.yaml

@@ -21,8 +21,6 @@ resources:
     state: exists
   - name: file4
     path: "/tmp/mgmt/f4"
-    content: |
-      i am f4 and i should not be here
     state: absent
 edges:
 - name: e1