Improve benchmarking tool

This commit adds several enhancements to benchmarking tool: server_url
and path to messages now can be configured in config file.  Default
output of program has been changed: now it prints values in json so they
can be parsed more easily. Previous human readable representation is
accessible via --verbose flag.

The `total_requests` metric now shows all performed requests (either
failed or successful) and new metric - `successful_requests` - was
introduced to store count of successful requests.

Change-Id: Id6fe4b2046394a348ba07eb5b2b003c6024b78b0
Partially-implements: blueprint gen-bench-reports
Merge "Improve benchmarking tool"