Add initial section for SambaFS vol driver

A SambaFS driver for cinder volumes was added in Juno.
This patch adds a stub section to the configuration reference
so its options can be easily found.

Change-Id: Ib08c839077ee52d380f7040e7f682ca6a78278bd
Closes-Bug: 1367345
Merge "Add initial section for SambaFS vol driver"