Better message if we dont find appropriate BlockStorage API

With latest devstack, v1 and v2 are DEPRECATED and v3 is marked
as CURRENT. So we fail to attach the disk, the error message is
shown when one does "kubectl describe pod" but the operator has
to dig into find the problem.

So log a better message if we can't find the appropriate version
of the API that we support with an explicit error message that
the operator can see how to fix the situation.

Note support for v3 block storage API is being added to gophercloud
and will take a bit of time before we can support it.

Author: dims

pkg/cloudprovider/providers/openstack/openstack.go

@@ -657,8 +657,10 @@ func (os *OpenStack) volumeService(forceVersion string) (volumeService, error) {
 		if autodetectedVersion := doBsApiVersionAutodetect(availableApiVersions); autodetectedVersion != "" {
 			return os.volumeService(autodetectedVersion)
 		} else {
-			// Nothing suitable found, failed autodetection
-			return nil, errors.New("BS API version autodetection failed.")
+			// Nothing suitable found, failed autodetection, just exit with appropriate message
+			err_txt := "BlockStorage API version autodetection failed. " +
+				"Please set it explicitly in cloud.conf in section [BlockStorage] with key `bs-version`"
+			return nil, errors.New(err_txt)
 		}
 
 	default: