add wmstats related information to reqmgr_workload_cache db

@zdenekmaxa, please review 
changed as you suggested from the review.
use reqmgr rest api instead of accessing couchdb directly (unit test added)
state transition validation occurs in python
consistent name for file.

This shouldn't be included until June upgrade. related deployment script change need to go in first
https://github.com/dmwm/deployment/pull/65