Do not run selinux commands if it is disabled.

 * Execution of the setenforce command when selinux was not enabled
   was resulting in a non-zero exit status, which halted devstack.
 * Addresses bug 1175444

Change-Id: I7f6492dea7c52d153e755cda826d6fabd53f3771
diff --git a/stack.sh b/stack.sh
index 7ec25cf..5dea000 100755
--- a/stack.sh
+++ b/stack.sh
@@ -565,7 +565,9 @@
 if [[ is_fedora && $DISTRO =~ (rhel6) ]]; then
     # Avoid having to configure selinux to allow things like httpd to
     # access horizion files or run binaries like nodejs (LP#1175444)
-    sudo setenforce 0
+    if selinuxenabled; then
+        sudo setenforce 0
+    fi
 
     # An old version (2.0.1) of python-crypto is probably installed on
     # a fresh system, via the dependency chain