upstart/ceph-osd: make crush update on start optional

By default, we will do some (minimal) CRUSH tree adjustment.  However,
we make this update optional.

Fixes: #3993
Signed-off-by: Sage Weil <sage@inktank.com>
upstart/ceph-osd: default crush weight of 0 for new devices

By default, when I plug a disk in, do not assign it data automatically.
This is more conservative, and more likely to be what users are after.
They can opt-in to the magic.

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