Add new NAS features as a separate service

This commit adds the 'shares' service to Cinder. Responding
to the concerns from the community, the code is significantly
refactored to have a minimal impact on the existing code.

This feature adds support for file-based storage in Cinder:
the NFS and CIFS. Along with shares API, this commit implements
a NetApp specific driver for CIFS and NFS access. The LVM driver
implementing same functionality will land in future.

Implements blueprint cinder-protocol-enhancements

Change-Id: I37cb542d13002687ecbfe7c7f3e0e7eb7cecbc03