bug: Add support for rack elevations API call

[pobradovic08]

Bug Report

Current behavior:

There is no way to call /api/dcim/racks/{id}/elevation/ API endpoint.
Similar situation to Available resources in IPAM (#8)

[pobradovic08]

I currently have it implemented like this: 4cc359f

Example call would be nb.dcim.racks.elevation.all(id=rack_id) (inline with option 1 in #10).

Also I'm unsure that ElevationAPIResource class should be named like this. Maybe it should have a more generic name as there is nothing specific to Rack Elevations and it could be used for other API endponts that provide additional information about some entity?

[arthanson]

@pobradovic08 can you open up a PR here?

[arthanson]

Fixed via #15