JetUserData -> read vertex collection only once per event

[kreczko]

Currently we are reading the vertex collection once per jet.
While CMSSW is smart about caching such access, it is a bad code example.
These two lines should move into the produce method

edm::Handle < std::vector<reco::Vertex> > primaryVertices;
iEvent.getByToken(vtxInputTag_, primaryVertices);

and the current fillVertexVariables method should take the vertex handle (const reference) instead of iEvent as paramter.