Hook support

[ferranbt]

There are some use cases that require generic pre/post hook support:

• Redis "join" command to create the cluster.
• Elasticsearch drain the nodes during scale down.
• Clickhouse replica distributed tables during scale up.

Even the generic concept of Readiness (after the start) could be modeled as a post hook. A hook should be a one-off operation. Long running jobs like monitoring are not applied here.

[ferranbt]

First iteration merged in #52