'/environments/<env_id>/model/<path>' endpoint added.
GET request responds with the subsection of <env_id>'s object
model located in its <path>.
PATCH request applies json-patch from request body to <end_id>'s
model. It does not contain <path> in the URL.
Change-Id: I672d43464ed7d5722cc574f1a10700b070664f34
Implements: bp environment-edit