Fix KeyError exception while updating dhcp port

This is caused by a missing network_id in the port body.
This patch adds it so that a warning message can be traced
correctly. Wording is slightly tweaked to ensure it applies
to the right context.

Closes-bug: #1324194

Change-Id: I4b67b5c3584aa33278eb0e9e879ca338ba0ca8b0
Merge "Fix KeyError exception while updating dhcp port"