hail: don't select the primary as new secondary

This just adds the primary node of the instance as 'non-allocable'
during the choosing of the new secondary.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>
(cherry picked from commit 7073b3a8)

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