ceph::key : fix cap_{mon,osd,mds} prototype to match the documentation

Set cap_{mon,osd,mds} to undef by default instead of '' and fix the
integration tests accordingly.

Change-Id: I520c3b4f1f983c8503fc7bac0a5bb05c90da0bbc
Signed-off-by: Loic Dachary <loic@dachary.org>