(#5605) Prefetch doesnt work with composite keys

The uniqueness_key method of a type or resource object should return a
key that can be used to identify this resource. In fact puppet seldomly
uses this method and instead uses resource[:name] as an identifier.

While this is totally fine for resourcetypes with a single
key_attribute (and resource[:name] returning the namevar), it breaks
things as soon as one creates a type with a composite key (prefetching
for example is broken). To ease the process of replacing calls to
resource[:name] to resource.uniqueness_key, the method uniqueness_key now
just returns name_var if there is only one key_attribute (immitating
self[:name]) and only returns an array of all the values of all the
key_attributes if we have more than one key_attribute.

The resourcehash which is passed to providers in their prefetch method
is now build with uniqueness_key as the hashkey. Because of the new
behaviour of uniqueness_key we hopefully wont break existing providers
while allowing new providers for types with composite keys to implement
correct prefetch methods.