API tests execution time reports

Added stage with benchmark tests for hivemind api calls.

• benchmark_generator.py will create set of benchmark tests basing on tavern.yaml tests in tests_api directory.
• Benchmarks will create json file which is parsed by json_report_parser.py. The html report is generated.
• Report contains data such as test name and mean execution time of a test (5 runs of each call).
• If mean time is greater than threshold the job running that test will fail.