retention_rules: error when applying rule to deleted workflow

[mdonadoni]

When a workflow is deleted, its workspace directory is deleted, too. This causes an error when applying retention rules, such as:

Applying rule <rule_id> to workflow <workflow_id>: 'path/to/delete/*' will be deleted from '/var/reana/users/<user_id>/workflows/<workflow_id>'
Error while applying rule <rule_id>: [Errno 2] No such file or directory: '/var/reana/users/<user_id>/workflows/<workflow_id>'
Setting the status of rule <rule_id> to `active`

We should handle this case in the script that applies retention rules, but we should also probably set the status of retention rules for deleted workflows to inactive.