Fixes #587. Notifiy when node is down for too long.

* Update notification NodeNotAvailable.
* Update offline time to 5 days.
* Handle flapping states.
Fix upgradecontroller, add backwards compatibility decode_version.
Fix setuptincd management command. Use dinamyc server ID.
Handle invalid and malformed build kwargs.
Fixes #594. Define mgmtnetworks permissions.
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
Add support to configure notifications expiration.

You can define the period while a delivered notification
remains valid. After that, notification becomes expired
and new emails could be sent.
Group notifications by set_state.
Migrate data: sliver_data XOR sliver_overlay (#200)
Cosmetics. Fix template image fields description.
Remove overlay fields of sliver (#200).
Move UCI mgmt_net.tinc_gateway -> node.tinc_gateway
Separate default_connect_to on hosts, nodes and servers (#236 note-42).
Merge branch 'feature236'
Set default_connect_to on existing hosts.
Merge branch 'master' into feature200

Apply feature236 changes into feature200.
Fix typo in warning about overlay settings removed.
Grant {Node|Server}Api base_uri always ends with a slash '/'.
Cosmetics. Fix layout and improve build change view.
Improve gateways dropped backwards compatibility. Fixes #236.

Keep a snapshot of current gateways to provide readonly api views
to keep backwards compatibility with old CNS.