Allow ShardAllocators to be configured via node level settings.

[s1monw]

since #2555 we have multiple options for ShardAllocators. We should give users the ability to select their implementation as well as one of the defaults impls.

[dakrone]

Are there any plans to allow additional weights to be added to shard distribution in the future?

For example, we created https://github.com/sonian/elasticsearch-equilibrium/ to work around ES' shard allocation because it was not taking shard sizes into account, causing some nodes in the cluster to run out of disk while others had plenty of disk. It'd be great if adding shard-size as a configurable weight was an option.

[s1monw]

hey @dakrone, further developments will likely add additional weights related to what you are mentioning. I personally would want to have this alg. to bake in a bit before we make it more complicated.