Elastic - Add support for ebean.docstore.generateMapping

[rbygrave]

So setting this means Ebean will generate mapping files for ElasticSearch indexes.

## generate mapping files for each of the indexes
ebean.docstore.generateMapping=true

Note that in development/testing you also frquently do:

## drop and re-create indexes
ebean.docstore.dropCreate=true

OR

## create any indexes that do not already exist
ebean.docstore.create=true