Add more the description for ProphetStor FC and iSCSI drivers

ProphetStor Fibre Channel and iSCSI drivers add support for ProphetStor
Flexvisor through OpenStack Cinder. ProphetStor Flexvisor enables
commodity x86 hardware as software-defined storage leveraging well-proven
ZFS for disk management to provide enterprise grade storage services such
as snapshots, data protection with different RAID levels, replication,
and deduplication etc.

Change-Id: I711049a50c16ae00363957747376207457fa602e
Closes-Bug: 1369415