User customizable template

[laduke]

We compile our template.network into the binary. It has good defaults.
We have some flags like -dns-over-tls=true to enable certain options in the template.

There are hundreds or thousands of options in systemd.network

I don't think we want to slowly add flags for each one.

So do something like:
zerotier-systemd-manager -template ./my-template

Now users can do whatever they want without asking for new code, new releases, etc...

Also, consider removing any existing flags that have to do with changing values in the template for consistency sake. (breaking change)