lb-agent: ensure removed devices get treated on resyncs

The previous exception handling in daemon_loop reprocessed all existing
devices as if they were new devices. However, it did not ensure that
any removed devices got cleaned up properly. This could result in
leftover security group iptables rules from deleted instances.

This patch refactors daemon_loop and scan_devices so devices that were
flagged for cleaning will get retreated in the next iteration if there
is an exception.

Change-Id: Ieada34ad315c0c29aa8462ebf041a448fde007b8
Closes-Bug: 1145610
Merge "lb-agent: ensure removed devices get treated on resyncs"