Add new configuration option in pipelines.yml to enable/disable pipelines

[agamagarwal101]

Having a configuration option in pipelines.yml to enable/disable pipeline using an environment variable will allow to use same set of configuration for different use-cases just by toggling few environment variables.
Example configuration:

pipelines.yml

- pipeline.id: pipeline-1 
  path.config: "/etc/logstash/conf.d/pipeline1.conf"
  
  pipeline.workers: ${PIPELINE_1_WORKERS:4}

  pipeline.enabled: ${PIPELINE_1_ENABLED:ENABLED}

- pipeline.id: pipeline-2
  path.config: "/etc/logstash/conf.d/pipeline2.conf"
  
  pipeline.workers: ${PIPELINE_2_WORKERS:4}
  
  pipeline.enabled: ${PIPELINE_2_ENABLED:ENABLED}