only mount cgroups if you haven't already

stack.sh

@@ -409,8 +409,10 @@ if [[ "$ENABLED_SERVICES" =~ "n-cpu" ]]; then
         if ! grep -q cgroup /etc/fstab; then
             echo none /cgroup cgroup cpuacct,memory,devices,cpu,freezer,blkio 0 0 | sudo tee -a /etc/fstab
         fi
+        if ! mount -n | grep -q cgroup; then
             sudo mount /cgroup
         fi
+    fi
 
     # User needs to be member of libvirtd group for nova-compute to use libvirt.
     sudo usermod -a -G libvirtd `whoami`