Description of problem:
--read-only option is ignored and volume being mounted with RW permissions.

Version-Release number of selected component (if applicable):
/usr/sbin/glusterfs --version
glusterfs 3.8.4 built on Aug  9 2017 17:54:51
Repository revision: git://git.gluster.com/glusterfs.git
Copyright (c) 2006-2013 Red Hat, Inc. <http://www.redhat.com/>
GlusterFS comes with ABSOLUTELY NO WARRANTY.
It is licensed to you under your choice of the GNU Lesser
General Public License, version 3 or any later version (LGPLv3
or later), or the GNU General Public License, version 2 (GPLv2),
in all cases as published by the Free Software Foundation.

How reproducible:
Always

Steps to Reproduce:
1. Mount /usr/sbin/glusterfs --read-only --volfile-server=SERVER --volfile-id=/VOLUME-FILE /mnt/mountpoint

Actual results:
df -h | grep mountpoint
SERVER:/VOLUME-FILE on /mnt/mountpoint type fuse.glusterfs (rw,relatime,user_id=0,group_id=0,default_permissions,allow_other,max_read=131072)

Expected results:
 df -h | grep mountpoint
SERVER:/VOLUME-FILE on /mnt/mountpoint type fuse.glusterfs (ro,relatime,user_id=0,group_id=0,default_permissions,allow_other,max_read=131072)

Additional info:
cat /etc/redhat-release 
CentOS Linux release 7.4.1708 (Core) 

uname -a
Linux ip-10-20-30-40 3.10.0-327.18.2.el7.x86_64 #1 SMP Thu May 12 11:03:55 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux