Backport sprint Hobart to `maintained` branches

Created by: fdobrovolny

As per https://github.com/SovereignCloudStack/issues/issues/387.

The following PRs were merged:

PR	maintained/v5.x	maintained/v5.0.x
#479 Fixes the handling of openstack auth_urls with implicit port	🌱	🌱
#476 Adds new environment regiocloud	🌱	❌
#474 Gets variable username from .tfvars	🌱	❌
#455 Rewrite documentation to be easier to get started.	🌱	❌
#454 Add ability to specify service and pod CIDRs	❌	❌

We should decide what should be backported if anything.