This will ensure that depending apps always have a root.key to work with.
The root.key will be seeded once when libunbound8 is installed/configured. If unbound is also installed, it will be kept current using RFC 5011 mechanism.
Closes: 900241, LP: #1771545
This is an simpler fix than what was proposed in !1 (closed) but has yet to be tested. I'll do proper testing if the main concept is deemed acceptable.