Use voluptuous for Russound RNET

[fabaff]

Description:
Migration of the configuration check to voluptuous.

Related issue (if applicable): fixes 127528299

Example entry for configuration.yaml (if applicable):

media_player:
  - platform: russound_rnet
    host: 192.168.1.10
    port: 1337
    name: Russound
    zones:
      1:
        name: Main Bedroom
      2:
        name: Living Room
      3:
        name: Kitchen
      4:
        name: Bathroom
      5:
        name: Dining Room
      6:
        name: Guest Bedroom
    sources:
      - name: Sonos
      - name: Sky+

[mention-bot]

@fabaff, thanks for your PR! By analyzing the history of the files in this pull request, we identified @laf to be a potential reviewer.

[laf]

I'll give this a test at the weekend if that's ok?

[balloob]

If this is required, do not default it to an empty list.

[fabaff]

Thanks.

[laf]

Sorry for the delay folks, tested and no issues. for me.