Ignore offline primary when failing over

When the source node for a failover is marked offline, there's no need
to require the user to specify “--ignore-consistency”.

To make it work at all, a number of bugs introduced by the merge of
migration and failover are also fixed by this patch.

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