Do not mark device as processed if it wasn't

Currently treat_devices_added_or_updated in the OVS agent skips
processing devices which disappeared from the integration bridge
during the agent loop.
This is fine, however the agent should not mark these devices as
processed. Otherwise they won't be processed, should they appear
again on the bridge.

This patch ensures these devices are not added to the current
device set.

The patch also changes treat_devices_added_or_updated. The
function now will return the list of skipped devices and not
anymore a flag signalling whether a resync is required.
With the current logic a resync would be required if retrieval
of device details fails. With this change, the function
treat_devices_added_or_updated will raise in this case and the
exception will be handled in process_network_ports.

For the sake of consistency, this patch also updates the
similar function treat_ancillary_devices_added in order to
use the same logic.

Finally, this patch amends an innaccurate related comment.

Closes-Bug: #1329546

Change-Id: Icc744f32494c7a76004ff161536316924594fbdb
Merge "Do not mark device as processed if it wasn't"