decorator for benchmark scenarios method

Added decorator @base.scenario in rally/benchmark/scenarios/base.py
The decorator adds extra field is_scenario = True to the methods
This is used to explicitly mark scenario methods as scenarios
Link to the trello Card
https://trello.com/c/xZjmtoQX/175-create-decorator-for-scenarios

Change-Id: Icbc182519b75d022ca2f538a9acfb7b8cd2729ad
Merge "decorator for benchmark scenarios method"