utils: Preferences loader - support non-versioned keys

Till today the preference loader treats keys and a node with
multi-version values on it

nodeA.nodeB.value.x = 1
nodeA.nodeB.value.y = 2

so "value" is also a node and x and y are keys on the node.

this is specifig behaviour for paths in the form of a.b.c.value.x.y.z

This design is not straight forward with non-versioned keys such as:

nodeC.key = value

so we can:

 Preferences nodeC = getNodeC();
 nodeC.keys() # ["key"]
 nodeC.get("key") # "value"

Bug-Url: https://bugzilla.redhat.com/1006340
Change-Id: I4b3bf67fbd892c37763428ae79204b59632d025d
Signed-off-by: Roy Golan <rgolan@redhat.com>