Add support for elasticsearch interface

[PrasadG193]

Fixes: #53

ELS configuration structure:

  communications:
    # Settings for ELS
    elasticsearch:
      enable: false
      server: 'ELASTICSEARCH_ADDRESS'              # e.g https://example.com:9243
      username: 'ELASTICSEARCH_USERNAME'
      password: 'ELASTICSEARCH_PASSWORD'
      # ELS index settings
      index:
        name: botkube
        type: botkube-event
        shards: 1
        replicas: 0

Helm command to deploy botkube with ELS support:

helm install --name botkube --namespace botkube \
--set config.settings.clustername=dev-gke \
--set config.settings.allowkubectl=true \
--set config.communications.elasticsearch.enable=true \
--set config.communications.elasticsearch.server=ELS_ADDRESS \
--set config.communications.elasticsearch.username=ELS_USERNAME \
--set config.communications.elasticsearch.password=ELS_PASSWORD \
helm/botkube/

[PrasadG193]

[sanketsudake]

@PrasadG193 Please make sure we also update documentation before this merge to master

[write2abhi]

How can we create indexes on a daily basis. Because It will generate a lot of data and we need to clear the data on daily basis?

[killcity]

Anyone else notice the cluster field not being populated in ES, but is correctly populated when calling Slack?

[codenio]

cluster field not being populated in ES

@killcity thanks for reporting this issue.
#164 fixes this.

[killcity]

@aananthraj Thanks!