openstack_dashboard/api nova.py and cinder.py are imported
by each other. To avoid cyclic imports, some imports are placed
inside functions now, but it would be nice if we can move them
to a separate module and nova.py/cinder.py can import it.

This commit tries to achieve this by moving some stuffs in nova.py
into a separate module named _nova.py.
The module name starts with an underscore to clarify
it is intended to use only in openstack_dashboard.api.

Change-Id: If91cf4f30d2ddab471757129c2a74b26b6d38b1e