Add update_cells to BandwidthUsage.create()

The switch from conductor_api.bw_update_usage to BandwidthUsage.create
caused a regression in reporting this information up to parent cells.
This adds the argument, and functionality, back in.

Change-Id: I495f9b5ed1845ad78c2d5da5b5f90b40a20c3ab3
Closes-Bug: #1392426
Merge "Add update_cells to BandwidthUsage.create()"