This fixes issue 396.

- Fix a wrong comment that mentions drbd8 when actually the code acts
  only on externally mirrored instances.
- Fix a wrong assert that requires failover/migrate to acquire the NAL
  on externally mirrored instances: this is the case only when a
  iallocator is used, not if a node is manually specified.

Extra qa should be added, but this can only be done if an externally
mirrored instance or a diskless instance is tested. It is easier to add
it on the master branch, where we have diskless instances support.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>