LU-12275 sec: control client side encryption

LU-12275 sec: control client side encryption

Client enables encryption by default. However, this should be
possible only if server side is encryption aware.
Moreover, we want to give the ability to decide which clients can
make use of encryption, by extending the nodemap mechanism with a
new 'forbid_encryption' property, set to 0 by default.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I765e5ce555e8277319c03c770cb6e6ac73cfc9e8
Reviewed-on: https://review.whamcloud.com/36433
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>