enable_firewall and enable_vpn settings have been deprecated
since Juno release, but they have not been dropped yet just because
we need a lot of changes in unit tests if dropped.
The situation is resolved by the parent commit and we can now drop these
settings safely. We can control whether a specific panel should be
enabled or disabled via the pluggable panel mechanism ("enabled" dir).
Also clean up enable_lb which has gone several releases ago.
Closes-Bug: #1687185
Change-Id: I83fa48c5d9568a00294be7cd5f43ef181bc1f28d
53a5164d82