Look for a DOCSIS ORO only if request is coming from a Cable Labs vendor

src/bin/dhcp4/dhcp4_srv.cc

    // Let's try to get ORO within that vendor-option
    /// @todo This is very specific to vendor-id=4491 (Cable Labs). Other
    /// vendors may have different policies.

This was the subject of a discussion here: !1235 (comment 213964)

It was probably not a review request, just a comment, but the solution was provided to leave no room for ambiguity. So I'm opening this issue to not lose track of these changes.

This seems to be a redundant change. Because the unit tests pass on master as well. But at least the unit tests would be a welcome change?