Migration advisory plugin

[clintongormley]

It would be useful to have a _site plugin that can be installed on 1.x, which would warn users migrating to 2.0 about the need to upgrade any indices, and any use they are making of deprecated features.

For instance, the tool would:

• use the segments API to look for 3.x segments - if found, it would recommend upgrading to 1.6 first, and running the upgrade API
• check mappings for any use of deprecated functionality in Remove possibility for conflicting field definitions and ambiguous field resolution #8870, and warn about the actions that will be taken in 2.0
• similarly, check index settings, analyzers, index templates, dynamic mappings etc
• possibly also deprecated cluster settings? (although won't have access to the yaml config file)

This tool won't be able to check use of deprecated query DSL, as that only happens at run time.

[clintongormley]

Still more tests to be added, but the migration plugin is already usable: https://github.com/elastic/elasticsearch-migration