Make elastic search configurable with yml file

[MadanThangavelu]

Configuration of endpoint of elastic search, the index names should be read form an external yml file. The yml file location should have the ability to be overridden using environment variable.

The current configuration is the default implied configuration by the elastic search client

pipelog/backend/elasticsearch.rb

Line 7 in 0112c55

@client = Elasticsearch::Client.new log: true