Update workflow execution data and resume after pause

Enable user to update the workflow execution context variables prior to resuming the workflow.

Use case 1) : workflow put in paused state due to error conditions (e.g. create VM failed due to network problems, workflow got "paused"). After resolving the problem, and fixing the conditions manually, user wants to resume workflow. (E.g., network restored, and VM is manually created). Now user needs to update the workflow execution context with new information (clean the error, and provide the vm_id) for workflow continuation.

Use case 2) manual intervention. As a user, i configure a task to pause the wrokflow on-complete, so I can provide input data to decide which path the workflow will take. E.g., "continue with deployment" or "stop".

Relevant to https://blueprints.launchpad.net/mistral/+spec/mistral-workflow-resume