[oauth2-proxy] Use remote chart dependency

[oauth2-proxy] Use remote chart dependency

- use remote chart dependency instead of versioning a copy of the whole chart in the OOM repo
- the copy was slightly modified to use repositoryGenerator.dockerHubRepository but that was
  the only change that was made [0]
- the huge benefit of not maintaining a copy in OOM is that we avoid having to do a diff when
  upgrading the chart
- even the knowledge that the chart is "custom" is a large mental barrier to upgrading, because
  it is unknown beforehand how big the diff may be

[0] there is nothing in this change to replace it right now, which means that an override for
    .Values.initContainers.waitForRedis.image.repository needs to be defined for this chart
    explicitly

Issue-ID: INT-2334
Change-Id: Ia7a2b9bc5db60f2a81be6b7bf9b93b005bf6cc6a
Signed-off-by: Fiete Ostkamp <fiete.ostkamp@telekom.de>