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>
"Override the setting of a particular attribute" is too misleading as it only works on global attributes.
Does adding one word, creating "Override the setting of a particular _global_ attribute", really make it a manual?