Allow region-ized fields in OpenStack CloudProfileConfig

[rfranzke]

What this PR does / why we need it:
Allow region-ized fields in OpenStack CloudProfileConfig. Some OpenStack environments have dedicated keystone URLs, floating pools or load balancer per region/data center. Today, you would have to create a dedicated CloudProfile per region (although the backing cloud provider is the same). That's unfortunate. With this PR, it's possible to configure regions for the mentioned properties. This will allow to have only one CloudProfile like for the hyperscalers.

Special notes for your reviewer:

• /cc @afritzler
• On purpose, I didn't change the type of KeystoneURL from string to *string (because it's optional now) to prevent vendoring issues. We could do that later if we want.

# github.com/gardener/gardener/pkg/apis/core/v1alpha1
vendor/github.com/gardener/gardener/pkg/apis/core/v1alpha1/conversions.go:580:34: cannot use cloudProfileConfig.KeyStoneURL (type *string) as type string in assignment
# github.com/gardener/gardener/pkg/apis/core/v1beta1
vendor/github.com/gardener/gardener/pkg/apis/core/v1beta1/conversions.go:507:34: cannot use cloudProfileConfig.KeyStoneURL (type *string) as type string in assignment
make: *** [start-provider-openstack] Error 2

Release note:

The OpenStack `CloudProfileConfig` now features a new `keyStoneURLs` field. It is a mapping of regions to keystone URLs. Also, the `constraints.loadBalancerProviders` and `constraints.floatingPools` fields now can optionally have a `region` property. This allows to only have one `CloudProfile` for an OpenStack environment in which the keystone URL, floating pools, and/or load balancer providers differ per region. See [this document](https://github.com/gardener/gardener-extensions/blob/master/controllers/provider-openstack/docs/usage-as-operator.md#cloudprofileconfig) for an example resource. :important: Please note that these fields will only be usable with gardener/gardener once the deprecated `garden.sapcloud.io/v1beta1` API group has been removed.

[afritzler]

Looks good to me, but I wanted to do some testing next week.

[rfranzke]

/cc @gardener/dashboard-maintainers - PTAL

[timuthy]

Nice PR! I have a general comment about the setting override, though.
For the keyStoneURL the non region specific setting overrides the region specifics and for LoadBalancerProvider and FloatingPool the order of elements determines how it is overwritten. I think this should be a) consistently handled and b) the region specifics should overrule the non-region specifics. WDYT?

[rfranzke]

As discussed with @timuthy I will adapt the PR so that it's allowed to specify both the non-regional and regional fields (e.g., keystoneURL and keystoneURLs). The logic will always prefer a regional setting if found, if not it will fallback to the non-regional setting.

[ialidzhikov]

Looks good to me.
Nit - rebase needed because of the new year.

[rfranzke]

Thanks, merging after rebase.