functional: simplify nspawn machine creation

Generating all SSH host keys every time we spin up a machine is
slow and unnecessary. We could consider moving to fixtures, but for now
it's easier to simply generate a single key.

systemd also introduced a hardware database service that was taking a
relatively considerable amount of time, and should be unecessary in a
systemd-nspawn container.

This brings the average machine bring-up to ~1s
test: drop schema from testables
Merge pull request #792 from jonboulle/testz

functional: simplify nspawn machine creation
engine/agent: change reconciler ticker behaviours

The engine/agent currently reconcile every $reconcileInterval,
irrespective of whether any reconcilations occurred for other reasons
(i.e. were triggered) within that period.

A better semantic is for the tick to occur every $reconcileInterval
_after the previous reconciliation was triggered_. To accomplish this,
reset the ticker every time a reconciliation is about to commence.
Merge pull request #798 from jonboulle/reconcilers

engine/agent: change reconciler ticker behaviours
Merge pull request #797 from bcwaldon/test-schema

test: drop schema from testables
unit: handle generator subscriptions earlier