gossip: allow multicast addresses for TVU

[alexpyattaev]

Problem

• Currently we block multicast addresses from getting specified for TVU, this makes multicast experiments with turbine impossible

Summary of Changes

• Allow multicast addresses for TVU
• Only allow it if port is set to 8002 (so all multicast group members use the same port)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 84.21053% with 3 lines in your changes missing coverage. Please review.

Project coverage is 82.8%. Comparing base (de54ce0) to head (c5b321c).

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #6415     +/-   ##
=========================================
- Coverage    82.8%    82.8%   -0.1%     
=========================================
  Files         848      848             
  Lines      379479   379484      +5     
=========================================
- Hits       314550   314511     -39     
- Misses      64929    64973     +44     

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