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.
Merge pull request #182 from dirkmueller/migration

Add Options related to Live-Migration