[FAB-2471] Fix gossip proto style 1/3

https://jira.hyperledger.org/browse/FAB-2471

This is the first in series of 3 changesets which normalizes the gossip
protos to use the official proto style guide.  These protos were missed
in the first sweep because the protos were not under fabric/protos.

Change-Id: Ifeef7c2bb9e1700ee16d8364ecf97a1da776ccc5
Signed-off-by: Jason Yellick <[email protected]>

Author: Jason Yellick