Fix @property_hash creation in custom types.

 * @property_hash for the network, subnet and router types was
   previously being created by referencing values in a hash indexed
   by strings, but using symbols to do so.  This patch ensures
   that strings are used to reference values in the hash.

Change-Id: I778ce277574d5f5b201b0173d51dfc16086e73cb
Merge "Fix @property_hash creation in custom types."