mon: OSDMonitor: don't expose uncommitted state on 'osd crush add/set'

Fixes: #4635

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
vstart.sh: s/osd crush set/osd crush add/ as it's supposed to be

'osd crush set' should only be used to update already existing items on
the map whereas 'osd crush add' should be able to 'add and update' items.

Considering at that point we are effectively adding a new item to the
crush map, use 'add' instead of 'set'.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>