Fixes #6297 - delayed jobs is dead, long live foreman-tasks

This pull request finishes the conversion of the async tasks based on delayed
jobs to the foreman tasks. The changes include:

* #6296 - dynflowize auto-attach
* #6304 - remove apply default info feature from the code for now
* #6182 - dynflowize repository update (thanks @aruzicka)
* #6297 - dynflowize content upload and after-sync callbacks
* #6297 - remove unused code around delayed jobs
* #6297 - updating the gemspec and rpm spec files to not use katello-jobs
          and use foreman-tasks service instead (already available
          in the foreman-tasks gem)
* #6283 - use dynflow API to figure out if the foreman tasks is running or not

Sorry for the longish pull-request but since the changes need some testing, it
might be easier to test all at once, than everything separately, when all we
need at the end is not having to use delayed jobs.

There should not be any changes in the behavior of the app, except
foreman-tasks service instead of katello-jobs, the ping returning
'foreman_tasks' instead of 'katello_jobs  and repo synchronizations and
package uploads tracked in foreman tasks.
I blieve this is already part of http://projects.theforeman.org/issues/6395 and we agreed on leaving it in the current state for 2.0, to lower the risk of regressions.