e2e-node tests may use custom system specs for validating nodes to
conform the specs. The functionality is switched on when the tests
are run with this command:

    make SYSTEM_SPEC_NAME=gke test-e2e-node

Currently the command fails with the error:

    F1228 16:12:41.568836   34514 e2e_node_suite_test.go:106] Failed to load system spec: open /home/rojkov/go/src/k8s.io/kubernetes/k8s.io/kubernetes/cmd/kubeadm/app/util/system/specs/gke.yaml: no such file or directory

Move the spec file under `test/e2e_node/system/specs` and introduce a single
public constant referring the file to use instead of multiple private constants.