Fix work of udp publisher

 * Signature is appended also to udp messages
 * All methods for convert from sample to message are moved to publisher/utils.py
 * In configs metering_secret is moved from group publisher_rpc to publisher
 * Changed tests for udp publisher

Fixes: bug #1259171

Change-Id: Ic7727c69d4be6d17ff00f21e323341efd57122a7
Merge "Fix work of udp publisher"