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
FWaaS plugin doesn't need to handle firewall rule del ops

If firewall rule is attached to firewall policy, it would raise
FirewallRuleInUse excpetion in DB ops, else it is a pure DB delete ops.
So it is useless to handle delete_firewall_rule ops in fwaas plugin.
Closes-Bug: #1322076

Change-Id: I55a000d206f232c79b41230f526007f684db8f4f
Merge "FWaaS plugin doesn't need to handle firewall rule del ops"
Merge "Fix KeyError exception while updating dhcp port"