BUG-509, BUG-808: Added datastore support for lists without keys.

The datastore needs to support users who do not specify a list key, but
how expect the item ordering to be retained.

Added test which test all three possible behaviours of list
statement: ordered map, unordered map and list without keys.

Change-Id: I9e136267f57a88e4d2a6a4476025c33497373518
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>
Signed-off-by: Robert Varga <rovarga@cisco.com>