Support custom actions to be triggered

[HonzaKral]

I'd like to see the ability for users to register their own actions to be run at given thresholds. Example API (rough draft):

@curator.action('close')
def close_index(client, index_name):
    client.indices.close(index=index_name)

Which can then be used via the CLI by specifying the 'close' action type.

Problem is how do we do the discovery - we could ask the users to wrap the code in their at this point:

#!/usr/bin/env python
import curator
# ... actions here
if __name__ == '__main__':
    curator.main()

or maybe just provide an env variable with a list of python modules to be loaded before the run:

CURATOR_ACTIONS='myapp.curator_actions' curator.py -....

This way curator itself wouldn't have to support all the options people might wish to run but will instead focus on selecting the indices for those actions and dispatching the calls to those actions.

[untergeek]

I believe this is now addressed with the API/script split in #159.

If not, we can re-open it and discuss for future releases.