Add 'project_id' to Execution and Task

 * this allows to filter executions and task
   by project (like it was done with workbooks,
   workflows and actions)

Partially implements blueprint mistral-multitenancy

Closes-bug: #1384552

Change-Id: I1e1cc338ebd594853f17a824cfabf0d9efa3a405
Merge "Add 'project_id' to Execution and Task"