Fix gnt-instance reinstall

Commit 55efe6da "Convert instance
reinstall to multi instance model" actually broke instance reinstall for
single-instance cases. This one-liner fixes it.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>
(cherry picked from commit b6e243ab)