Support auto-enrollment and configuration file for ziti-edge-tunnel

[sabedevops]

It would be useful to be able to automatically enroll identities. This would reduce friction when deploying ziti-edge-tunnel via configuration management. For example, a new flag -I to match the run command's:

ziti-edge-tunnel enroll -I /path/to/identities

The statement above could be used in the ExecStartPre section of the systemd service file if made idempotent.

Additionally, it would simply service unit startup procedure if ziti-edge-tunnel supported a configuration file:

IdentityFile: /path/to/id.json
IdentitiesDir: /path/to/identities
AutoEnrollTokens: true
UseMultipleIdentities: true  

By default, ziti-edge-tunnel could read /opt/ziti/etc/ziti-edge-tunnel.yaml
or from a new cli flag (-c) that accepts a path.