Change ebean artifact such that ebean-ddl-generator is a dependency of ebean-test and ebean-autotune optional

[rbygrave]

That is DDL generation is generally only required when running tests hence ebean-ddl-generator is a dependency of ebean-test (and not ebean).

Also change such that people looking to use the optional autotune functionality explicitly include the ebean-autotune dependency.

    <!-- now a dependency of ebean-test only -->
    <dependency>
      <groupId>io.ebean</groupId>
      <artifactId>ebean-ddl-generator</artifactId>
    </dependency>

    <!-- optional and not included by default -->
    <dependency>
      <groupId>io.ebean</groupId>
      <artifactId>ebean-autotune</artifactId>
    </dependency>