Allow multiple loadBalancerProviders as the default and for a region

[lotharbach]

How to categorize this PR?

/area networking /kind enhancement /platform openstack

What this PR does / why we need it: We are adding a new openstack region that uses the OVN networking stack, and also the octavia OVN loadbalancer provider that has some worthwhile advantages. But since OVN is also still missing features we want to also allow the well known amphora provider in parallel.

Currently the cloudprofile/controlplane validation does not allow multiple entries for a region.

The Shoot spec validation has always required to specify a loadBalancerProvider even when there is only a single choice, the behavior for a given shoot should stay the same, and the behavior with existing cloudprofiles should also be the same until you add multiple loadBalancerProviders.

Which issue(s) this PR fixes: Fixes #862

Release note:

Allow multiple loadBalancerProviders as the default and for a region