fix for _get_external_device_interface_name trace

fix for _get_external_device_interface_name trace

On removal of external gateway from DVR the code path
external_gateway_removed(...) was trying to access the
agent gateway port interface even when no fip namespace exists.
This change checks for existence of namespace before
accessing the agent gateway port interface through
_get_external_device_interface_name(...) or looking for
floating ips that may have been configured on the
port.

Change-Id: Idcf28ff93f16f1d692fe7acb678fb90aabe5af5e
Closes-Bug: #1421497