Adds ability to boot a server via the Nova V3 API

Creates an images client when attached to the the servers
client. This is necessary because the Nova V3 API no longer
proxies image queries to glance but when preparing a request to
boot a server it is necessary to retreive information about
images so we need to talk to both Nova and Glance in the same
command.

This is a bit ugly, but not much more than the already existing
ugliness of using the client class designed to talk to
Nova to talk to Glance and Cinder. The long term clean solution
is probably to a unified client that is designed to talk to
multiple openstack services.

Differences between the V2 and V3 API are described here:
https://wiki.openstack.org/wiki/NovaAPIv2tov3

Partially implements blueprint v3-api

Change-Id: Ib43682f38cd7a3e0f910b75e96685591246e7f67