Move update_assembly_status function from worker to conductor

Creates a new method in conductor specifically to update the current
status of an assembly. Remaining in the shell and shell_nobuild workers
are two uses of solum.objects.registry for looking up assembly and plan
data.

Change-Id: I5c8a6d8a2f83748d6d51605d9935fbeb8459afac
Closes-Bug: #1388163
Merge "Move update_assembly_status function from worker to conductor"