cfg: clean up None value handling
cfg: clean up None value handling

Remove the need for an internal NoneValue class by making the existence
of the 'default' or 'override' keys signify whether a default or
override is set.

Change-Id: Iacf49553df5ba8414307904a3ee334c7b8c55758

JioCloud/oslo-incubator,JioCloud/oslo-incubator,JioCloud/oslo-incubator