Fix e2e tests for release branches

[embik]

Which jobs are failing?: pre-dashboard-test-e2e across all active release branches is flaky and for release/v2.20 doesn't seem to pass at all.

Which test(s) are failing?:

1) DigitalOcean Provider
       should verify that there are no clusters:
     CypressError: Timed out retrying after 600000ms: `cy.wait()` timed out waiting `600000ms` for the 1st request to the route: `default-6`. No request ever occurred.

1) KubeVirt Provider
       should wait for the cluster to be ready:
     AssertionError: Timed out retrying after 600000ms: Expected to find element: `.cluster-name`, but never found it.

Among others. Check job history, it appears that various tests are failing: https://public-prow.loodse.com/job-history/gs/prow-dev-public-data/pr-logs/directory/pre-dashboard-test-e2e.

Since when has it been failing?:

Unclear, has been a problem for several releases.

Prow link to failing test:

https://public-prow.loodse.com/job-history/gs/prow-dev-public-data/pr-logs/directory/pre-dashboard-test-e2e

Reason for failure:

Failing test cases

Anything else we need to know?:

/

[Waseem826]

I've marked E2E tests optional in release/v2.19 and release/v2.20 branches to smooth the release process a bit. I'm not closing this issue in case we want to further investigate and fix flaky tests.