@Override
    public ApplicationPackage applicationPackage() { return applicationPackage; }
The Version of Vespa this model should specify that nodes should use. Note that this
is separate from the version of this model, as upgrades are not immediate.
We may build a config model of Vespa version "a" which specifies that nodes should
use Vespa version "b". The "a" model will then be used by nodes who have not yet
upgraded to version "b".
configserver/src/main/java/com/yahoo/vespa/config/server/deploy/ModelContextImpl.java