Raising exception for invalid floating_ip's ID

The proposal is to validate the ID from the request to check
whether it is is valid for nova-network driver.
If the network driver is nova-network, the ID should be numeric.

Change-Id: I10df4f9ba2f5cfec7a6d2112cf5905944a9b5419
Closes-bug: #1078055
Merge "Raising exception for invalid floating_ip's ID"