Add template systemd service files

[wpyoga]

These allow hosts to easily join multiple different Nebula networks.

Also, I have removed the SyslogIdentifier=nebula option, it's unnecessary because the logs already get tagged nebula by default.

[wpyoga]

I have added a commit (to the same files) that makes nebula run inside the respective config directories. This enables this kind of configuration:

pki:
  ca: ca.crt
  cert: host.crt
  key: host.key

Which makes it easier to write the config files. No need to write something like key: /etc/nebula/MyNetwork2/host.key.

[IngwiePhoenix]

Would love if someone looked into this, this'd be very handy!