Use novaclient version 2 instead of 3
Use novaclient version 2 instead of 3

The novaclient library has removed client version 3 (see
https://review.openstack.org/#/c/169378/).  This commit updates
get_nova_client() method in maas_common.py to use version 2 instead.

Closes issue #177

cloudnull/rpc-openstack,jacobwagner/rpc-openstack,sigmavirus24/rpc-openstack,byronmccollum/rpc-openstack,darrenchan/rpc-openstack,xeregin/rpc-openstack,BjoernT/rpc-openstack,claco/rpc-openstack,mattt416/rpc-openstack,git-harry/rpc-openstack,miguelgrinberg/rpc-openstack,shannonmitchell/rpc-openstack,prometheanfire/rpc-openstack,stevelle/rpc-openstack,prometheanfire/rpc-openstack,robb-romans/rpc-openstack,miguelgrinberg/rpc-openstack,xeregin/rpc-openstack,robb-romans/rpc-openstack,galstrom21/rpc-openstack,mattt416/rpc-openstack,andymcc/rpc-openstack,rcbops/rpc-openstack,claco/rpc-openstack,mancdaz/rpc-openstack,sigmavirus24/rpc-openstack,andymcc/rpc-openstack,rcbops/rpc-openstack,major/rpc-openstack,darrenchan/rpc-openstack,nrb/rpc-openstack,busterswt/rpc-openstack,xeregin/rpc-openstack,sigmavirus24/rpc-openstack,nrb/rpc-openstack,major/rpc-openstack,cfarquhar/rpc-openstack,shannonmitchell/rpc-openstack,jacobwagner/rpc-openstack,git-harry/rpc-openstack,miguelgrinberg/rpc-openstack,busterswt/rpc-openstack,cloudnull/rpc-openstack,cfarquhar/rpc-openstack,claco/rpc-openstack,darrenchan/rpc-openstack,nrb/rpc-openstack,stevelle/rpc-openstack,hughsaunders/rpc-openstack,stevelle/rpc-openstack,BjoernT/rpc-openstack,mattt416/rpc-openstack,galstrom21/rpc-openstack,byronmccollum/rpc-openstack,andymcc/rpc-openstack,byronmccollum/rpc-openstack,xeregin/rpc-openstack,busterswt/rpc-openstack,darrenchan/rpc-openstack,hughsaunders/rpc-openstack,sigmavirus24/rpc-openstack,mancdaz/rpc-openstack