Parents should be frozen after creation.
Parents should be frozen after creation.

Use a tuple to ensure that the parents list
can not be mutated after flow creation.

Change-Id: Ic2a0ed73a2a7211184deb92d57720ff417429dca

jimbobhickville/taskflow,openstack/taskflow,junneyang/taskflow,varunarya10/taskflow,varunarya10/taskflow,jimbobhickville/taskflow,citrix-openstack-build/taskflow,pombredanne/taskflow-1,jessicalucci/TaskManagement,pombredanne/taskflow-1,junneyang/taskflow,openstack/taskflow,jessicalucci/TaskManagement,citrix-openstack-build/taskflow