Add a container balancer tool or service for HDDS

When an existing Ozone cluster is nearly full, we have to add more datanodes into the Ozone cluster, but there are two issue we must face.

• When new allocate container request coming, SCM should better to choose the datanodes in low usage, if not, the performance will getting pool.
• For read request, the existing datanodes stored lots of blocks, so they are responsible for serving the read request and supply the data stream service, meanwhile, the new coming datanodes can help nothing.

If we have a balancer tool just like hdfs balancer, we can move the block or container from some high usage datanodes to low, I think this is one of necessary tools for Ozone.

container balancer design doc https://docs.google.com/document/d/15PdYaP6aLB18ptbcOK3XWlL4Y1PLfIP-ll30KjNPZ2g/edit?usp=sharing