Add checksum field to wf definition

Problem description:
We have no way to determine whether the workflow definition has changed or not. The only method is the updated at. This complicates the work with Mistral for external services.

Solution:
Add 'checksum' field to workflow_definitions_v2 solve this issue.

Description:
1. checksum field added to workflow_definitions_v2 table in mistral/db/v2/sqlalchemy/models.py
2. the value is filled in mistral/services/workflows.py using hashlib for created and updated wf def.
Note: old wf def keep empty checksum