Allow scenarios to just ensure that an entity exists

Use the 'Given a Dummy entity "dummy"' syntax for entities where only
the identity of the entity matters.

Note that existing entities will not be updated, so this does not
guarantee that the entity will have default values.
Support "entity" as well as "entities" for table of entity fields

To specify existence of a single entity with a complex set of fields,
the syntax "Given the following Foo entity" is preferable to using
"Given the following Foo entities".