EA and ES Cluster

[buratinopy]

Hello!
The EA code is hardcoded to connect to only one ES node
elastalert/init.py [19]: host=conf['es_host']
config: "es_host": "es_standalone_host.corp"

We need to be able to connect to the ES cluster with the following config:

"es_hosts": [
   "es_cl0_n0.corp",
   "es_cl0_n1.corp",
   "es_cl0_n2.corp",
]

For compatibility in the operation of both methods, it is enough to change 2 lines in the file elastalert/init.py:

        super(ElasticSearchClient, self).__init__(host=conf.get('es_host', None),
                                                  hosts=conf.get('es_hosts', []),

246aa14

[buratinopy]

ready to make a pull request

[jertel]

Your PR is welcomed, please ensure all contribution guidelines are followed for the quickest merge.

Also, for background context, please review Yelp/elastalert#3109.