Add selinux check to lorax-composer

anaconda needs to have SELinux set to disabled or permissive in order to
run correctly. Check at startup and exit with an error.

src/sbin/lorax-composer

@@ -28,6 +28,7 @@ import argparse
 import grp
 import os
 import pwd
+import selinux
 import sys
 import subprocess
 from threading import Lock
@@ -178,6 +179,9 @@ if __name__ == '__main__':
     except KeyError:
         errors.append("Missing group '%s'" % opts.group)
 
+    if selinux.is_selinux_enabled() and selinux.security_getenforce():
+        errors.append("selinux must be disabled or in Permissive mode.")
+
     # No point in continuing if there are uid or gid errors
     if errors:
         for e in errors: