Tempest's scheduler_available_filters has a special 'all' value that
is understood to mean 'all filters are enabled' by various tempest
tests. However, what it really means is 'the default nova filters are
enabled.' In an effort to help clean that up, this patch explicitly
sets scheduler_available_filters to nova's $FILTERS. Because $FILTERS
is now used in both lib/nova and lib/tempest, it is renamed
$NOVA_FILTERS.
Change-Id: I6ffc1e9989cd61d666f9c1db9c94fbabd7151918
Related-bug: 1628443