mon/OSDMonitor: disallow crush buckets of type 0

Prevent creation of buckets of type 0 ('osd', 'device', etc.), as they
will confusing the mapping algorithm.

Signed-off-by: Sage Weil <sage@inktank.com>
test: merge unittest_crushwrapper and unittest_crush_wrapper

Signed-off-by: Sage Weil <sage@inktank.com>