Add -n option to mount.ceph.  Required by autofs when /etc/mtab is a link to /proc/mounts (e.g. Debian Wheezy), otherwise automounting a ceph file system fails.  Also useful when /etc is read-only.  feature 7006

Signed-off-by: Steve Stock <steve@technolope.org>
Merge pull request #942 from sstock/master

Add -n option to mount.ceph, feature 7006

Reviewed-by: Sage Weil <sage@inktank.com>