LU-12275 sec: enable client side encryption

LU-12275 sec: enable client side encryption

Enable client side encryption. By default it is activated,
letting user specifies actual encryption policy to use on
a per-directory basis. It is possible to deactivate client
side encryption by using the 'noencrypt' mount option.

Also add the test dummy encryption mode option to ease
testing.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I0e8d4db7ab8a77aba0600788cca9403f7c50f8a6
Reviewed-on: https://review.whamcloud.com/36143
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>