Fix the issue which raise FloatingIpPoolNotFound

This patch adds the necessary explanations on how to
use floating-ip, and add a parameter "pool" to fix the issue
which raises FloatingIpPoolNotFound when using nodepool.

Change-Id: I2344acd8b49f990eeb02c1b6016d1869f9d665f5
Closes-Bug: #1245829
Merge "Fix the issue which raise FloatingIpPoolNotFound"