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
Search for Glance-server in the right environment
Don't mount shared storage if compute and controller are shared

We're just exporting the very same filespace.
Merge pull request #196 from dirkmueller/glance_search

Search for Glance-server in the right environment
Merge pull request #182 from dirkmueller/migration

Add Options related to Live-Migration
Add library search_env_filtered

Provides a search that automatically applies an "env_filter", i.e. doing
a search against the correct proposal. To make this work,
nova/db/database_instance had to move to nova/database_instance (in line
with _all_ other *_instance entries in JSON files).

As a nice side effect, most previously broken env_filter get fixed
automatically. Before, some search() invocations re-(ab)used env_filters
from previous searches for entirely different barclamps. This should
help re-supporting multi-proposal in the future.

This approach should work for other barclamps too, but with Crowbar's
current architecture, each would have to ship it's own copy of this
library. A follow-up commit could move this to a more appropriate
(shared) location.