Fix conditional docker_opts in salt/docker/docker-defaults

'is defined' apparently returns true if the variable exists but has empty/None value. In that case you get

`DOCKER_OPTS="${DOCKER_OPTS} None"`

in /etc/default/docker, which causes the docker command to fail. Fix is to use [plain if](http://jinja.pocoo.org/docs/dev/templates/#if) , which checks that variable is defined and 'nonempty'.