Allow instance disc activation with offline secondaries

Currently, this is not allowed, so one can't run a replace-disks; this
breaks any non-invasive method of recovering the redundancy of the
instance if its disks are already stopped (but it still works if the
disks on the primary are active). So let's fix this inconsistency.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>

lib/cmdlib.py

@@ -6374,10 +6374,12 @@ def _AssembleInstanceDisks(lu, instance, disks=None, ignore_secondaries=False,
                                              False, idx)
       msg = result.fail_msg
       if msg:
+        is_offline_secondary = (node in instance.secondary_nodes and
+                                result.offline)
         lu.proc.LogWarning("Could not prepare block device %s on node %s"
                            " (is_primary=False, pass=1): %s",
                            inst_disk.iv_name, node, msg)
-        if not ignore_secondaries:
+        if not (ignore_secondaries or is_offline_secondary):
           disks_ok = False
   # FIXME: race condition on drbd migration to primary