(Feature) Use multiset in spec.json instead of safeContract

[igorbarinov]

Problem:
Hard forking of validators contract requires modification of code if use structure like

        "validators": {
          "safeContract": "0xf472e0e43570b9afaab67089615080cf7c20018d"
        }

https://github.com/oraclesorg/oracles-scripts/blob/master/spec.json#L9

Solution:
use multiset feature
https://github.com/paritytech/parity/wiki/Validator-Set#multi-set

Concerns:

• need to test it with hardfork on Sokol