Change 'entity' command to JPA-specific 'entity jpa' command

Change 'entity' command to JPA-specific 'entity jpa' command. This is to allow other persistence options to be supported and integrated.

Since the Roo 1.2.0.M1 release Roo supports different persistence stores including relational (through JPA) and non-relational (through Spring Data). To allow a more coherent way of creating domain entities (persistence backed types) the current 'entity' command needs to be adjusted to 'entity jpa'. This will allow other entity implementations to be created in the same project (ie 'entity mongo', 'entity graph', and others).