Add single_use_slave coverage to the apply tests

Currently the single_use_slave class is not tested by the apply tests
because there is no code path from a long-living puppet node to this
class, so create a dummy node definition for the purpose of adding a
path to this class in the apply tests.

Change-Id: I4ff4f1e42107b6df7ee7e4c845faa37d0f97ea65
Merge "Add single_use_slave coverage to the apply tests"