Add doc related to log level

Add doc related to log level to enable gophercloud request logs.

Author: hidekazuna

docs/book/src/clusteropenstack/configuration.md

@@ -11,6 +11,7 @@
   - [DNS server](#dns-server)
   - [Machine flavor](#machine-flavor)
 - [Optional Configuration](#optional-configuration)
+  - [Log level](#log-level)
   - [External network](#external-network)
   - [Floating IP](#floating-ip)
   - [Network Filters](#network-filters)
@@ -120,6 +121,10 @@ The flavors for control plane and worker node machines must be exposed as enviro
 
 # Optional Configuration
 
+## Log level
+
+If with `--v=6` the gophercloud client would log it's requests. This would really help during debugging.
+
 ## External network
 
 External network is automatically found, but you can specify the external network explicitly by `spec.externalNetworkId` of `OpenStackCluster`.