contint: get redis under /mnt/redis

The redis class takes care of creating the directory so we do not need a
symlink from /var/lib/redis to /mnt/redis. That even produce a duplicate
definition error:

 File[/var/lib/redis] is already defined in file
/etc/puppet/manifests/role/ci.pp at line 275;
cannot redefine at /etc/puppet/modules/redis/manifests/init.pp:38

Change-Id: Ia5ce396795f1d1d0fb8de572c1bee558a712f34b