Assigning ACL ID to network_acl_id  instead of route_table_id

In the endElement method, in NetworkAclAssociation, the networkAclId is getting assigned to the variable self.route_table_id instead of self.network_acl_id. NetworkAclAssociations should not have a route_table_id variable.
Adding unit test for pull request #2548
Merge pull request #2548 from cshanes/patch-1

Assigning ACL ID to network_acl_id  instead of route_table_id. Fixes #2548.