Add connection to headscale

[GrigoriyMikhalkin]

metal-api should be able to connect to headscale server. For that it should be able to accept headscale-addr flag.

cc @majst01 @Gerrit91 In case headscale address is provided metal-api will be adding VPN for all projects. But what if headscale server will go down for some reason? IMO, it should fail to create new firewalls/machine in that case with explicit error that headscale server is down. What do you think?

[majst01]

Yes if headscale server is configured but down, machine/firewall creation must fail

[Gerrit91]

Yep, please make it conditional --> if headscale server is configured it fails, if it's not configured everything works as before