We need to actually wait for the "Install" button to be enabled before clicking it otherwise we have a race condition where it sometimes works. Also change the `expect(page.find(...))` to `expect(page).to have_css` as this is prefered to avoid race conditions

Also remove the flakiness by using
`expect(page).to have_css(..., exact_text: '')` instead of
`expect(page.find(...).text).to eq('')` since the former will wait until
the text has this value while the latter will only wait for the element
to exist then test its text straight away which introduces a race
condition if the element is already there with a previous text value.
This test was failing locally most of the time because of this other
way of asserting.

Removes double quoted string

Part of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4110

Add test for checking interchangeability between KubernetesService and Clusters::Platform::Kubernetes