Add Docs: Elasticsearch Security Privileges

[abhishekjiitr]

Add new page in documentation specifying the Elasticsearch Security Privileges required for ElastAlert

Closes #2384

[jmcclelland]

Thanks for this patch - it is very helpful. I'm brand new to elasticsearch so I don't have much context to make meaningful suggestions, other than insight as to what a newbie will be confused by.

One simple suggestion is to spell out Role Based Access Control (RBAC).

Also, I now finally understand the what Search Guard is (I have been using x-pack security and could not understand why those permissions didn't match the documentation for x-pack, which I thought was the only option for security in elasticsearch).

If you want to include instructions for x-pack here is some verbiage you are free to use, discard or edit:

X-Pack Permissions

If you are monitoring an index called "journalbeat_*" and elasticalert is writing back to indices prefaced with "elastalert_" - the following role definition will grant the appropriate permissions to the elastalert role:

elastalert:
  cluster: ["monitor"]
  indices:
    - names: [ 'journalbeat-*' ]
      privileges: [ 'read'  ]
    - names: [ 'elastalert_*' ]
      privileges: [ 'all' ]

[nsano-rururu]

Merged into elastalert2
jertel/elastalert2#39