BugLink: http://bugs.launchpad.net/bugs/1670508

This allows users to inforce encryption for SMB3 shares if a server
supports it.
Signed-off-by: Pavel Shilovsky <pshilov@microsoft.com>
(backport from commit ae6f8dd4)
Signed-off-by: Joseph Salisbury <joseph.salisbury@canonical.com>
[cascardo: fixup of conflict with 06e7bc14]
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>