Cluster verify: check for missing bridges

Currently cluster verify doesn't check for bridge information; the
only checks are done at instance create and failover/migrate
time. This means a cluster that seems healthy will fail creation jobs.

This patch implements a simple verification that all nodes (in the
entire cluster, so doesn't work well for multi-group) have all the
required bridges: the default one plus any instance bridge.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>