Allow choose default tinc gateway on node firmware configuration.

* Extend tinc with default_connect_to attribute.
* Include new UCI entry to store tinc_gateway.
* Store tinc gateways on /etc/confine/tinc-gateways
Move UCI mgmt_net.tinc_gateway -> node.tinc_gateway
Separate default_connect_to on hosts, nodes and servers (#236 note-42).
Merge branch 'feature236'