[JBERET-84] Remove static instances of repository types from jberet-core

@chengfang I could use a review on this.

Removed static instances of ```JobRepository``` types. These could be a problem in an environment like WildFly that can be reloaded with new environment specific objects. For example the ```JdbcRepository``` could hold on to a ```DataSource``` that is no longer valid.

Make the ```JobRepository``` environment specific by making it part of the ```BatchEnvironment``` SPI.