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
Merge "Do not run selinux commands if it is disabled."