Add a service plug-in for data transfer among nova nodes

Nova has implemented the native functions for some hypervisors to do live
migration, e.g. libvirt, VMware VCenter, etc. Shared storage(NFS) and
block(iSCSI) migration have been implemented as well. To meet the need of
migrating data(like VMs) between the machines within one network as the
first use cases mentioned above, this is fairly enough. To transfer huge data
from one machine to another in a different network or outside the firewall, in
which case we are unable to establish the NFS or iSCSI connection, other
transfer protocols, like FTP, need to be taken into account.

This proposal will abstract a data transfer plug-in for the transfer protocols
to implement. Any protocols, like NFS, iSCSI, ftp, bitTorrent, etc, can be put
in this module for nova to use. BitTorrent,

Furthermore, FTP will be taken to transfer data between two machines in
different networks fill up the second use case. BitTorrent, which has been
implemented for image download in Xen, can be implemented as well for
transferring the same data among many machines simultaneously.