Add encoding argument to deserialising udp packets in collector

Samples from udp publisher failed signature verifying when they
contain utf8 strs. It happened because msgpack.loads
unpacked data without utf8 by default and in compute signature
this strs are processed as different.
This is fixed by adding of encoding in unpacking packets from
udp

Fixes: bug #1297731

Change-Id: I3a8072eb49a61364ff1a6bff1a14ad2c9925b88e
Merge "Add encoding argument to deserialising udp packets in collector"