Add Nova Cells support

Bring nova-cells support into puppet-nova.

Cells is a feature introduced in Grizzly and improved in Havana.
This patch configures nodes to be part of a parent or child
cell.

It also adds "it_configures" in spec_helper.

Work in progress: do not merge.

Implements: blueprint cells-support
Change-Id: I10b6a0080cfbefeddf6a19fc661a3ddfaf3da218
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>