Handle IpAddressGenerationClient neutron

Handles IpAddressGenerationClient neutron exception which
can occur when allocating a floating ip fails because
the pool has been exhausted. Convert the exception
to a nova NoMoreFloatingIps exception which will be handled
by the api layer and converted to a 404.

Change-Id: Ib5320a13b5cc0c493ef0763d0e2b4575dc699bdf
Partial-Bug: 1210598
Merge "Handle IpAddressGenerationClient neutron"