Do not default Octavia provider to "octavia" (#2208)

In Octavia the names of the providers are completely free-form and up to
cloud administrator. While the standard names are "amphora" (aliased as
"octavia") and "ovn", we cannot be guaranteed these names will work with
every OpenStack cloud.

This commit makes sure we do not default the Provider name to "octavia"
when creating an LB for the ingress-controller as this may not work for
every cloud. Instead when option is unset we will pass an empty value
causing Octavia to use whatever is configured as the cloud's default.

I also added docs explaining that option a bit better.

Author: dulek

docs/octavia-ingress-controller/using-octavia-ingress-controller.md

@@ -152,6 +152,13 @@ Here are several other config options are not included in the example configurat
       flavor-id: a07528cf-4a99-4f8a-94de-691e0b3e2076
     ```
 
+- Option to set which Octavia provider to use. If unset octavia-ingress-controller will leave it unset so the load balancers will be created with the default provider configured for that OpenStack cloud, decided by the cloud administrator. You can use `openstack loadbalancer provider list` to check available Octavia providers. Please note that currently only Amphora provider is supporting all the features required for octavia-ingress-controller to work correctly. That provider can be named differently in the cloud you use.
+
+    ```yaml
+    octavia:
+      provider: amphora
+    ```
+
 ### Deploy octavia-ingress-controller
 
 ```shell

pkg/ingress/controller/openstack/octavia.go

@@ -291,18 +291,11 @@ func (os *OpenStack) EnsureLoadBalancer(name string, subnetID string, ingNamespa
 			return nil, fmt.Errorf("error getting loadbalancer %s: %v", name, err)
 		}
 
-		var provider string
-		if os.config.Octavia.Provider == "" {
-			provider = "octavia"
-		} else {
-			provider = os.config.Octavia.Provider
-		}
-
 		createOpts := loadbalancers.CreateOpts{
 			Name:        name,
 			Description: fmt.Sprintf("Kubernetes ingress %s in namespace %s from cluster %s", ingName, ingNamespace, clusterName),
 			VipSubnetID: subnetID,
-			Provider:    provider,
+			Provider:    os.config.Octavia.Provider,
 			FlavorID:    flavorId,
 		}
 		loadbalancer, err = loadbalancers.Create(os.Octavia, createOpts).Extract()