osd/OSDMap: add 'chassis' to default type hierarchy

A chassis is usually bigger than a host but smaller than a rack.  This will
be useful for a broad class of modern hardware that sticks multiple hosts
in the same chassis (in sleds, or on cards, or blades, or whatever).

Signed-off-by: Sage Weil <sage@inktank.com>
osd/OSDMap: add region, pdu, pod types while we are at it

One use noted that they have a 'pdu' type in their type hierarchy that
typically spans multiple racks.  Others are known to use the 'pod'
terminology; add that to.  And I can imagine 'region' above datacenter.

Factor this into a helper to make things a bit less fragile.

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