1096931 - improving repo update command to better detect spawned tasks

Moved constants around so that they can be used by both client and server code.

Conflicts:
	server/pulp/server/async/tasks.py
	server/test/unit/server/async/test_tasks.py
Merge pull request #982 from pulp/mhrivnak-1096931

1096931 - improving repo update command to better detect spawned tasks