config: re-add murmur3_ignore_msb_bits to scylla.yaml
config: re-add murmur3_ignore_msb_bits to scylla.yaml

Commit d6b0c4dda40cfffd1482a5681bf941ec4f5e805d changed the built-in default
murmur3_ignore_msb_bits to 12 (from 0) and removed the scylla.yaml default.

Removal of the scylla.yaml default was a mistake for two reasons:
 - if someone downgrades a cluster, keeping scylla.yaml derived from the
   master branch, they will experience resharding since the built-in default,
   which has changed, will take effect. While that scenario is not supported,
   it already happened and caused much consternation.
 - if, in the future, we wish to change the default, we will cause resharding
   again. Embedding the default in scylla.yaml allows us to change the default
   for new clusters while allowing upgraded clusters to retain older values.

Therefore, this patch restores murmur3_ignore_msb_bits in scylla.yaml. Future
changes to the configuration item should change both scylla.yaml and the
built-in default.

Message-Id: <89be84dab19e6a260dd1ddd39391635fe35fc6dc@scylladb.com>

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