Add domain field to `openstackv1alpha1.FloatingPool`

[rfranzke]

What would you like to be added: Let's add a new optional domain *string field to the openstackv1alpha1.FloatingPool. It allows to configure that the given floating pool is only available in the specified domain. If no domain is specified then it is valid in all domains:

apiVersion: core.gardener.cloud/v1beta1
kind: CloudProfile
metadata:
  name: openstack
spec:
  type: openstack
  ...
  providerConfig:
    apiVersion: openstack.provider.extensions.gardener.cloud/v1alpha1
    kind: CloudProfileConfig
    ...
    constraints:
      floatingPools:
      - name: fp-pool-1 #valid in `domain1` domain in `eu-1` region
        region: eu-1
        domain: domain1
      - name: fp-pool-2 # valid in all domains in `eu-1` region
        region: eu-1
      - name: fp-pool-3 # valid in all domains and all regions
      loadBalancerProviders:
      - name: haproxy

Why is this needed: In some OpenStack environments the floating pools differ based on region (already supported) and domain (not yet supported). Reflecting this in the CloudProfile will make the user experience better and allows the Gardener Dashboard to provide the correct values in the cluster creation dialog.

[rfranzke]

/cc @RaphaelVogel @mandelsoft @grolu @petersutter

[rfranzke]

/in-progress /area usability