Add wait for feature flag in iterations e2e spec

What does this MR do and why?

Fix for flaky create_iterations_spec. The consensus is that the applications settings cache isn't being updated for 60 seconds which is causing flakiness in some tests on some environments, most notably this one. The reason we need to sleep instead of checking the UI is because the functionality is sporadically available until the cache is refreshed as documented here: gitlab-org/quality/testcases#113 (comment 300647725) so the test progresses because the elements were found in the UI but fail on the next step.

Also removes an unused requires_admin tag.

This should theoretically fix these errors:

#343645 (closed) #346114 (closed) #347337 (closed) #350429 (closed) #350545 (closed) #352699 (closed)