Check for floating IP pool in nova-network

There are no checks in nova-network to see if a specified
floating IP pool exists or not before attempting to allocate
an IP address. If a pool does not exist, nova throws an
unrelated error - 'No more floating ips available'. The
following patch adds a check for the existence of the IP pool
and throws a FloatingIpPoolNotFound if it is not found.

Change-Id: I72cb8dd044f7ac094287bad7df9167b8e77a4b5b
Related-Bug: #1355623
Merge "Check for floating IP pool in nova-network"