Add MethodNotAllowed and Conflict exception classes

It is expected to recieve a 405 or a 409 from Nova, but novaclient presents
them as a generic ClientException. This patch adds MethodNotAllowed
and Conflict classes to represent these HTTP responses.

Change-Id: If89cee04ebd74daaa28ea30b5c57cdc63edc1551
Merge "Add MethodNotAllowed and Conflict exception classes"