Add description support for execution

Currently, mistral doesn't support description field for execution, when user creates an execution, he/she could not identify that execution only if he/she remember the execution id, which is not a user friendly way (the execution id doesn't have any meaning at all).

This proposal will add a new field 'description' for execution database table, with empty string as default to keep backward compatible. At the meanwhile, user could specify this information when he/she creates execution like below:

execution-create wf1 --description my_first_workflow_execution