properly separate persistence specific and general things

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | see travis
| Fixed tickets | -
| License       | MIT
| Doc PR        | TODO

This will validate the configuration to actually tell you when you try to use the bundle without any provided or custom object mapper. Moving all the configuration that is not storage dependent out of the perstistence-phpcr config file.