Allow the use of NFS files as virtual block devices

This blueprint creates a Cinder extension to allow guest volumes to be stored on NFS shares. Each volume maps to a single file on the NFS. Cinder config file will point to separate file which contains the list of the shares. List of shares can be updated at run time. Driver will reread the list of shares if the file is modified. Driver will load balance file creation, choosing the share with the most available space to create the new volume file. Using NFS as a storage for volume files gives users additional flexibility in selection a data storage solution. NFS servers can be built into a highly scalable and reliable storage pool. This driver will support generic Linux NFS server implementation.

Proposed use-case:
1. user sends create-volume command
2. cinder-volume component selects the NFS with the most free space
3. cinder-volume component mounts selected NFS share and creates file for the volume (file name should match the volume name or index).
4. user sends attach-volume request
5. nova compute volume driver mounts NFS share and attaches given file to the VM as a disc storage.
New volume driver is added to Cinder and nova-compute volume driver is modified:
1. new volume driver for cinder is added to support NFS Linux server.
2. nova-compute volume driver is updated to dynamically mount appropriate NFS shares on the compute host.