Support to import tasks from group refs

This change adds support to import a single task from any group
ref (All-Users.git:/refs/groups/..) by specifying the group name or
group uuid in task config file.

See documentation for usage details.

Release-Notes: tasks can now be imported from group refs
Release-Notes: a task reference cannot start with '%' unless referring to a group
Change-Id: Ie0b346acff17de14768676ca0c64b61f8d345610