disassociate the nova id from the ovz id

The driver no longer relies on the deprecated nova 'id' column
in order to configure openvz. Some additional changes were made
to associate containers with the nova host that spawned them
to allow this driver to run multiple on nova-compute instances on
a single host.

A side benefit is now we have an object representing an openvz
container that can be extended to provide better OO facilities
than were previously available.

Change-Id: Ie86f4133f9a2e0afdaff805870855b12abd87a18