simplify 'get-service-fn' signature
simplify 'get-service-fn' signature

Previously, it accepted an arbitrary sequence of keys (since graph
can handle arbitrary nesting like this).  However, that doesn't
correspond to our model of services with a flat list of service
functions.

nwolfe/trapperkeeper,rbramwell/trapperkeeper,camlow325/trapperkeeper,senior/trapperkeeper,scotje/trapperkeeper,senior/trapperkeeper,puppetlabs/trapperkeeper,nwolfe/trapperkeeper