Don't always run etcd scaling jobs in openshift/origin PRs

When the etcd scaling presubmits were added to openshift/origin (https://github.com/openshift/release/pull/34552/files) the associated jobs were set to always_run: true for every PR.
This however clouds the CI signal on origin PRs as those jobs are always subject to invariant test failures from issues across the platform.

Since those jobs are optional and non-blocking for origin PRs they should be set to always_run: false. We already have enough coverage with the scaling test being blocking on etcd-operator and CPMSO repos.
https://github.com/openshift/release/blob/651097f1dd0802f550d6776334bc2dac6eff8030/ci-operator/jobs/openshift/origin/openshift-origin-master-presubmits.yaml#L740

This should be done for all jobs/platforms and backported to 4.13 and 4.12