ceph-disk: s/else if/elif/

Signed-off-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Joao Luis <joao.luis@inktank.com>
(cherry picked from commit bd8255a750de08c1b8ee5e9c9a0a1b9b16171462)
(cherry picked from commit 9e604ee6943fdb131978afbec51321050faddfc6)