Clarify nwfilter not found error message
Clarify nwfilter not found error message

In bug #1039398 the user got an error message about an instance's
nwfilter not being found, but it gave the impression that live
migration was involved.

The ensure_filtering_rules_for_instance() method was originally
just used for live migration, but since:

  https://code.launchpad.net/~chemikadze/nova/driver-agnostic-restart-instances/+merge/69069

it has also been used when re-starting the compute service when
there are running VMs.

Clarify the error message and the comments in the code to reflect
this.

Waiting up to 30 seconds before reporting an error here seems
pretty bad. It appears this is to handle the case where the
nwfilters get defined in a separate thread. For reference, the
code was added by this merge:

  https://code.launchpad.net/~nttdata/nova/live-migration/+merge/44940

For now, just add a comment explaining why we're polling for
the existence of the nwfilter.

Change-Id: Ieb085a6753ac72116d5f5a706fd1a908703286ad

n0ano/ganttclient