output flannel vars as docker command line options

Add two new options:

-docker-fmt=false
-docker-fmt-prefix=DOCKER_NETWORK_OPTIONS

These will switch from

    FLANNEL_SUBNET=1.2.3.4/8
    FLANNEL_MTU=1450

to:

    DOCKER_NETWORK_OPTIONS="--bip=1.2.3.4/8 --mtu=1450"

So the docker unit file can import the env file and use it as docker
command line options.