Modify list_sizes method in the OpenStack driver to include
OpenStackNodeSize object which includes 'vcpus' attribute which holds
a number of virtual CPUs for this size.

Contributed by Carlo, part of LIBCLOUD-325.


git-svn-id: https://svn.apache.org/repos/asf/libcloud/trunk@1482230 13f79535-47bb-0310-9956-ffa450edef68