Fix signature validation failure when using qpid message queue

When using qpid message queue, the metering message will get
signature validation failure because of extra unicode encoding
given by json loads. This change peels off unicode of the message
so that this issue can be avoided.

Change-Id: Ia23af789460ad8597867902f713b8a78f4a09e06
Closes-Bug: 1373356
(cherry picked from commit 4166da70b30291e8de74cf8c18e27753d0902ace)
Merge "Fix signature validation failure when using qpid message queue" into stable/juno