Allow to publish several counters in a row

Support publishing of multiple counters at once. This change the pipeline
and RPC calls to send a list of counter by default instead of only once
counter. This allows to make only one RPC calls when sending multiple
counters to be more efficient.

This implements blueprint publish-counters-list-rpc

Change-Id: Ie4155b35585f261e6ff9816e5a845a479151eefd
Signed-off-by: Julien Danjou <julien@danjou.info>