crush: support chooseleaf_vary_r tunable (tunables3) by default

Add TUNABLES3 feature (chooseleaf_vary_r tunable) to a set of features
supported by default.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>