cluster/afr: Avoid self-healing extended attribute used by SELinux.

Since removexattr() fails to remove "security.selinux" in a system 
where SELinux is enforcing, xattr self-healing fails. 
As a consequence of this, user extended attributes are not being healed.
Added a check in afr to prune SELinux xattr from the dictionary 
used for removing xattrs from the sink.

Minor changes in tests and md-cache as well.

Signed-off-by: Vijay Bellur <vbellur@redhat.com>
Change-Id: I854bfc0098dde812ce2afe64b125ee40c04bdeb1
BUG: 957877
Reviewed-on: http://review.gluster.org/4905
Reviewed-by: Venky Shankar <vshankar@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>