crush/CrushWrapper: explicitly set tunables on create, decode

Set the tunables to the 'default' values on create.

Default to the legacy values when we decode, so that an old map without
values defined gets legacy values.

Signed-off-by: Sage Weil <sage@inktank.com>
crush/CrushWrapper: default to the "new" bobtail-era tunables

These are supported as of kernel version v3.9:
 - chooseleaf_descend_once was added in 1604f48
 - others were added in 546f04e

And of course these are supported by bobtail and later.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #888 from ceph/wip-crush-tunables

default to bobtail-era crush tunables.

Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>