Changes EBS attachments to be initiated from NC instead of CLC. Fixes EUCA-5179.

Changes EBS message flow signifcantly and adds a new required network path from each
NC to the SCs in the cluster regardless of which backend the SC is using.

Prior to this commit the CLC would call attachVolume() on the SC an pass it the
list of IQNs for each NC in the cluster for BfEBS instances or the single specific
NCs IQN for EBS volume attachments. The SC returned the connection string and the
CLC would pass it down to the NC(s) via the CC.

This commit adds the concept of an attachment token to the EBS architecture. A single
EBS attachment (for BfEBS or regular EBS) has an associated token that is requested by
the CLC prior to the call to the NC to do the attachment. That token is opaque and sent
to any/all NCs involved in the attachment. Each NC then calls the ExportVolume() operation
on the SC to resolve the token and get a host-specific connection string used to connect
the EBS volume to the NC host.

This commit adds:
1. New SOAP interface for SC to NC using a new XML namespace: storagecontroller.eucalyptus.com
2. ExportVolume/UnexportVolume operations to be invoked by the NC on the SC
3. GetVolumeToken operation invoked by the CLC on the SC to get a new token for a volume attachment
4. A C client to make calls to the SC from the NC. And the associated eucalyptus_sc.wsdl
5. New VolumeToken and VolumeExportRecord entities in eucalyptus_storage db to persist tokens and exports
6. Refactoring of ebs volume operations in the NC into the single ebs_utils.c/h module to isolate ebs ops