Check for missing network in _bind_devices

Check for missing network in _bind_devices

_bind_devices was making the assumption that the ports it
was operating had local VLAN map entries for their network.
This wasn't the case when a network was deleted right before
_bind_ports was called because the VLAN was reclaimed.

This patch just checks to see if the the network ID has an entry
in the map. If not, it skips the port. The port will be handled
on the next scan_ports iteration when the agent will discover that
the port is no longer defined on the plugin and it will be placed
in the DEAD vlan.

Change-Id: Ica51d727aceb41848fec0f4edbd16916365941ee
Closes-Bug: #1452903