Restructured DCfgMgrBase to group the top level elements in a configuration
into scalars (strings, bools, ints, etc...) and objects (maps, lists, etc),
and parse the scalars first, then objects.  This permits the top level
scalars to be treated as a group of global parameters that are parsed first.

Ordered parsing is now relegated to only object elements. Scalars are parsed
first before any objects.

Also added the ability to reset config manager's context and rather than
than starting configuration parsing with a copy of the current context, it
starts with an empty context.

Modified unit tests accordingly.