Docs: update OpenStack documentation

[Fedosin]

This PR updates OpenStack documentation based on recent changes.

1. Updates Swift requirements. Now Swift is not a mandatory service but just an optional backend for image registry.
2. Adds Glance requirements. Glance is used for storing RHCOS images and bootstrap ignition configs.
3. Adds RHCOS image customization section for disconnected environments.
4. Moves "Cluster destruction if its metadata has been lost" section from "known-issues" to "troubleshooting".

[mandre]

You left an uppercase S in there, it can't find the anchor.

[Fedosin]

done :)

[mandre]

/approve

[iamemilio]

Are we supposed to be exposing the image overrides to the customer? My understanding was that these are not supported, and should not be recommended to customers.

[Fedosin]

@iamemilio I did this because the way we use the variable is a little bit unusual and I got a lot of PMs where people ask to explain how this works. So I wanted to have it documented somewhere and give them the link instead of explaining it over and over again.
Also, you can't install OpenShift on a disconnected environment without setting this variable, so users should be aware of this possibility.

[iamemilio]

Hmm that does seem problematic, however I think I would like alignment from @sdodson before approving this.

[sdodson]

/approve
I'm fine with the custom image stuff being in dev docs. Don't know if you wanted to squash any of this down or not.

[openshift-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mandre, sdodson

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[iamemilio]

/lgtm

[openshift-bot]

/retest

Please review the full test history for this PR and help us cut down flakes.

[openshift-bot]

/retest

Please review the full test history for this PR and help us cut down flakes.

[openshift-ci-robot]

@Fedosin: The following tests failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
ci/prow/e2e-aws-fips	2f6af25	link	/test e2e-aws-fips
ci/prow/e2e-aws-scaleup-rhel7	2f6af25	link	/test e2e-aws-scaleup-rhel7

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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