Add Options for Shared Storage and Migration

Add UI for setting up Shared Instances Storage on
Nova-Multi-Controller that is mounted on the Compute
Nodes. Currently it defaults to NFS (v3), which seems
to be the easiest to set up.
Implement NFS export of instances on Nova-multi-controller

NFS export the /var/lib/nova/instances directory from
Nova-Multi-controller. This is mounted by the Compute Nodes.
Enable Live-Migration in Libvirt

Configure libvirt on the compute nodes to allow live
migration over the admin network. Set Live-Migration
flag in nova.conf when Migration is enabled.
Attributize the hardcoded /var/lib/nova/instances
Also disable shared storage again when configured to do so
Don't mount shared storage if compute and controller are shared

We're just exporting the very same filespace.