Fix the DockerCon EU 2018 hang

[simonferquel]

This was caused by a very sneaky timing involving

• A stack deletion
• A machine going to sleep at a very bad time
• Etcd compaction happening just after resume

This resulted on the controller somehow missing a "delete" event on a stack, and attempting to get its owner information in a loop to do some kind of reconciliation

When we detect that an owner is requested for a stack that is not present and where its owner is not cached, we let the process panic, to let the controller be re-scheduled and re-initialized properly with a full sync

[simonferquel]

Fixed validate bug, and also added a commit to test a flakky e2e test related to view/edit/admin generic roles

[codecov-io]

Codecov Report

Merging #50 into master will increase coverage by 0.03%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
+ Coverage   66.97%   67.01%   +0.03%     
==========================================
  Files          82       82              
  Lines        8446     8444       -2     
==========================================
+ Hits         5657     5659       +2     
+ Misses       2557     2552       -5     
- Partials      232      233       +1

Impacted Files	Coverage Δ
internal/controller/stackreconciler.go	75.82% <100%> (+6.59%)	:arrow_up:
internal/controller/stackownercache.go	92.06% <100%> (+0.26%)	:arrow_up:
internal/controller/resourceupdater.go	55.55% <100%> (+0.19%)	:arrow_up:
internal/convert/stack.go	85.04% <100%> (ø)	:arrow_up:
internal/registry/owner.go	85.71% <0%> (-14.29%)	:arrow_down:
internal/registry/stackregistry.go	68.25% <0%> (-3.18%)	:arrow_down:

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