Add get_vlan_tag_by_id()

Change-Id: I314f7fcdbf19b7f4fdcc5466236a3e8305291a2a
diff --git a/db/db.py b/db/db.py
index e1a0126..0b50420 100644
--- a/db/db.py
+++ b/db/db.py
@@ -407,6 +407,13 @@
     def get_vlan_name_by_id(self, vlan_id):
         return self._get_element("vlan_name", "vlan", "vlan_id", vlan_id)
 
+    # Simple lookup: look up a VLAN by ID, and return the tag of that
+    # VLAN.
+    #
+    # Returns None on failure.
+    def get_vlan_tag_by_id(self, vlan_id):
+        return self._get_element("tag", "vlan", "vlan_id", vlan_id)
+
     # Grab one row of a query on one column; useful as a quick wrapper
     def _get_row(self, table, field, value):