Remove the dynflow configuration from Katello

Since, it's part of foreman-tasks plugin, it doesn't make much
sense to configure it from the Katello.

The remote executor is turned on by default in procution mode, can
be changed by setting `ForemanTasks.dynflow.config.remote`.

The remote executor can be run with:

```
rake foreman_tasks:dynflow:executor
```

Depends on https://github.com/iNecas/foreman-tasks/pull/4