Change 0797e82aca0a408838a49c1a269948e83d978b1c[1][2] fixed the ip
version used by ovs-agent but this effectively changed the ip version
used for underlying network. However that change did not update
the overlay_ip_version option accordingly and neutron records the error
caused by version mismatch continuously.
```
Invalid input for operation: ('Tunnel IP version does not match ML2
overlay_ip_version ...)
```
This change fixes that and ensures the option is set according to
the actual IP version.
Change-Id: Ied95886a30618ae2901cb2e9eacf9296a5911e8f
9dc6df9e75