Add encoding to keys in compute_signature

Unlike fixed sample keys as counter_name and counter_volume,
metadata keys may contain unicode characters.
ceilometer.publisher.utils.compute_signature can't calculate
signature of unicode symbols and crashes.
In this patch encoding keys is added.

Change-Id: I94d02ce20999510e9c6aaf33f7ee503bbb350e87
Closes-bug: #1391517
Merge "Add encoding to keys in compute_signature"