feat(es-7-migration): Update script and migration docs and add a simple Dockerfile

[dexter-mh-lee]

The script requires ssl configuration even if we disable ssl. Update the script to take optional disable-ssl params for source and destination.

Add simple Dockerfile to run a docker container that runs the script.

Update migration doc with recommendations on how to migrate to es7.

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable)

[dexter-mh-lee]

@shirshanka @jplaisted Please take a look! Let me know if the migration plan needs more details.

[shirshanka]

LGTM.
@jplaisted any modifications you would like to see?

Maybe branch name should be called elastic-5-legacy ?

[jplaisted]

I'm fine with whatever name for the branch. Naming is hard, bikeshedding probably not worth it :p

@shirshanka did you want to cut a release before ES7? Maybe a 0.6.2 the immediately a 0.7.0? Up to you, I'm not sure if there are other breaking changes other than ES, if not then this would let people upgrade as far as possible without going to ES7.

[dexter-mh-lee]

+1 to having two releases. If we decide on the version numbers, I can add that into the readme as well.

[jplaisted]

I think if there's no other major breaking changes other than ES7 then 0.6.2 and 0.7 will work. Otherwise 0.7 and 0.8 (where 0.8 is ES7, ironically). If its too hard to tell we can just do 0.7 and 0.8.