[REVIEW ONLY] service & environment fields

Added dictlike field *AssetEnvType* to models.
Use it in *ralph.discovery.device*.
No migration file yet.

@quamilek @xorxor @ar4s
* Name 'service' will be looks better,
* Let the field shows the ``proxy model``, which returns only the ``CI`` from ``CMDB`` with the type of service,