Truncate encoded instance message to 255

In nova/compute/utils.py function exception_to_dict truncates unicode
message to 255. However, in non-English locales, instance message may
be longer than 255 after encoding the unicode message to byte message.
Need to truncate the encoded byte message to 255, in order to ensure
db insert operation succeed.

Change-Id: I70cd2d6e4590ea95510c1ea3d51287f0dea14956
Closes-Bug: 1389102
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()"
Merge "Truncate encoded instance message to 255"