Cycles in task graph should report which tasks loop

[Matthew Mosesohn]

Detailed bug description:
 If a plugin developer writes an invalid task that creates a loop in the task graph, it is hard to identify the soruce.

Steps to reproduce:
 Quite difficult to explain, but create two tasks that require each other.
Expected results:
 Error message should say which tasks are in a loop.
Actual result:
 Unhelpful message in Fuel UI: "Tasks can not be processed because it contains cycles in it."
Reproducibility:
 Always
Workaround:
 Write correct tasks, which isn't always simple.
Impact:
 Benefit for developers of tasks to identify errors, rather than getting blocked.
Description of the environment:
 Default options, detached-keystone plugin, for example
Additional information:
 N/A

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/298325

[OpenStack Infra (hudson-openstack)]

Change abandoned by Matthew Mosesohn (<email address hidden>) on branch: master
Review: https://review.openstack.org/298325
Reason: abandoning in favor of https://review.openstack.org/#/c/297335/10

[Vladimir Kuklin (vkuklin)]

Fixed by https://review.openstack.org/#/c/297335/

[ElenaRossokhina (esolomina)]

Verified on 9.0 so#327:
cat /etc/fuel_build_id:
 327
cat /etc/fuel_build_number:
 327
cat /etc/fuel_release:
 9.0
cat /etc/fuel_openstack_version:
 mitaka-9.0
rpm -qa | egrep 'fuel|astute|network-checker|nailgun|packetary|shotgun':
 fuel-release-9.0.0-1.mos6344.noarch
 fuel-provisioning-scripts-9.0.0-1.mos8691.noarch
 network-checker-9.0.0-1.mos72.x86_64
 fuel-mirror-9.0.0-1.mos135.noarch
 fuel-openstack-metadata-9.0.0-1.mos8691.noarch
 fuel-notify-9.0.0-1.mos8362.noarch
 fuel-ostf-9.0.0-1.mos934.noarch
 python-fuelclient-9.0.0-1.mos315.noarch
 fuelmenu-9.0.0-1.mos270.noarch
 fuel-9.0.0-1.mos6344.noarch
 fuel-utils-9.0.0-1.mos8362.noarch
 fuel-nailgun-9.0.0-1.mos8691.noarch
 rubygem-astute-9.0.0-1.mos743.noarch
 fuel-library9.0-9.0.0-1.mos8362.noarch
 shotgun-9.0.0-1.mos88.noarch
 fuel-agent-9.0.0-1.mos277.noarch
 fuel-ui-9.0.0-1.mos2684.noarch
 fuel-setup-9.0.0-1.mos6344.noarch
 nailgun-mcagents-9.0.0-1.mos743.noarch
 fuel-misc-9.0.0-1.mos8362.noarch
 python-packetary-9.0.0-1.mos135.noarch
 fuel-bootstrap-cli-9.0.0-1.mos277.noarch
 fuel-migrate-9.0.0-1.mos8362.noarch

I've created two fake tasks required each other and error message looks alike:
Error
Deployment has failed. Method task_deploy. Cluster[]: Loop detected! Path: Task[first_fake_task_for_yaql/1], Task[second_fake_task_for_yaql/1], Task[first_fake_task_for_yaql/1].