Dynamically generate gateway external address in dvr models

Currently reclass classify command fails to generate gateway
external ip dynamically, because it's hardcoded in reclass,
this patch fixes this in the same way as it was done for OVS
models.

Change-Id: Id838d05b8bb5bba19ad1566769ffdd07bf5c0771
Related-Prod: https://mirantis.jira.com/browse/PROD-20157
6 files changed