HPCC-2566 Allow attribute values to be specified on the command line

Also allow ^ as a synonym for the root scope.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
I imagine the typical uses will be

-Dmydef=xxx
or
-Ddefinitions.myval=xxx

where definitions is a user-determined module which groups all the command line definitions.