Adds ExternalIpAddressExhaustedClient exception

Adds the ExternalIpAddressExhaustedClient exception so
a more specific exception than NeutronClientException is raised
when IPs run out. This will allow for better exception
handling in Nova.

Closes-Bug: 1233143

Change-Id: I9f40bf14eec0f485dfd21b36605a2474b99a8941
Merge "Adds ExternalIpAddressExhaustedClient exception"