Fix Flannel network on CoreOS

    Resolves: #748

roles/network_plugin/flannel/templates/flannel-options.conf.j2

 [Service]
+{% if ansible_os_family == "CoreOS" %}
+Environment="DOCKER_OPT_BIP=--bip={{ flannel_subnet }} --mtu={{ flannel_mtu }}"
+{% else %}
 Environment="DOCKER_NETWORK_OPTIONS=--bip={{ flannel_subnet }} --mtu={{ flannel_mtu }}"
+{% endif %}