Adds IpAddressGenerationFailureClient exception

Adds the IpAddressGenerationFailureClient exception so
a more specific exception than NeutronClientException is raised
when IPs run out. This will allow for better exception
handling in Nova and help fix nova bug 1212137

Change-Id: I921ba5d5f5ee3dc9620dcdbbd04d6fef824c9a53
Closes-Bug: 1231836