Sets default service type for Nova V3 API

Set the default service type when talking to the Nova V3 API to
computev3 rather than compute. Although it is rather ugly to have
a different service type for a different version of a service,
this is necessary for the medium term because traditionally the
compute service endpoint has pointed to the V2 API rather than
the root and then version discovery done.

This change allows progression of the V3 API support in
novaclient and although devstack too currently points computev3
directly to the V3 API, the intent is to change this to point to
the root and implement version discovery during the Icehouse
development cycle.

Longer term when the V2 API support is eventually removed we can
reclaim the 'compute' service type and point it to the root.

Partially implements blueprint v3-api

Change-Id: If5d6a0d8af037cde7bf253d71aac2823b89f8066
Merge "Sets default service type for Nova V3 API"