fix when we use is defined without != '' in ansible roles

marathon_artifact_store_dir is defined and marathon_artifact_store_dir != ''
/workspace/apollo/roles/consul/tasks/main.yml:
57: when: consul_join is defined
71: when: consul_join is defined

/workspace/apollo/roles/docker/tasks/main.yml:
37: when: http_proxy is defined and http_proxy != ''

/workspace/apollo/roles/marathon/tasks/main.yml:
11: when: marathon_artifact_store_dir is defined

/workspace/apollo/roles/mesos/defaults/main.yml:
35: {% if mesos_master_quorum is defined %}

/workspace/apollo/roles/serverspec/tasks/main.yml:
48: when: test_role is defined and serverspec_run_tests

/workspace/apollo/roles/zookeeper/defaults/main.yml:
10: {%- if zookeeper_host_list is defined -%}

/workspace/apollo/roles/zookeeper/templates/zoo.cfg.j2:
6: {% if zookeeper_conf is defined %}