default ebean server is now configurable

This pull request addresses the following problem:

Unit Testing EBeans using different databases 

In Dev I want to use MySQL and for testing I want to use H2. But doing that turns out to be very hard for following reasons:
  - If you are using "default" in dev mode then you cannot use "default" in test because it will override the evolutions.
  - if you create a new one in test then there is no way easy way to make it a default EBean server.