Fix bashisms found in swarm template fragments.

Partial fix for problems output by checkbashisms for file
magnum/templates/swarm/fragments/write-docker-service.sh only.

Quoted variable as recommended.

Change-Id: Iadcd3435b39ad7bd2575bbf838add1c22b04944f
Partial-Bug: #1561232

magnum/templates/swarm/fragments/write-docker-service.sh

@@ -23,7 +23,7 @@ ExecStart=/usr/bin/docker daemon -H fd:// \\
           -H tcp://0.0.0.0:2375 \\
 END_SERVICE_TOP
 
-if [ $TLS_DISABLED == 'False'  ]; then
+if [ "$TLS_DISABLED" = 'False'  ]; then
 
 cat >> /etc/systemd/system/docker.service << END_TLS
           --tlsverify \\