EbeanPlugin: Add support for read-only data-sources

Originally from https://github.com/playframework/playframework/issues/3237

Currently (Play 2.2, Play 2.3) when using multiple data-sources then it is not possible to disable evolutions and DDL automatic generation. This is usually not a problem, but becomes a problem with at least one of the sources being read-only.

There is a PR for disabling evolutions for specific data-sources, but there is no way to disable DDL generation per data-source. Only way is to disable EbeanPlugin DDL generation globally like this:

evolutionplugin=disabled
This does not solve the problem, because we want evolutions for non-read-only datasources.

Currently, when evolutionplugin is enabled (the default behavior) then evolution scripts are generated automatically by using DDL extraction here. The scripts are created and created and since this is a read-only datasource then they cannot be applied.

There should be a data-source specific option to disable automatic DDL generation.

PS! The option name evolutionplugin is also misleading for this particular use-case.

Start at the EbeanPlugin configuration and the DDL extraction path described in the issue, then trace how the evolutionplugin setting is applied across data-sources. Determine where a data-source-specific option can control automatic DDL generation while leaving evolutions enabled for other sources. Done means read-only sources no longer generate unappliable DDL scripts and other sources retain their existing evolution behavior.