Limit instance fault messages to 255 characters

The instance_fault.message column is a VARCHAR(255).  By default,
MySQL silently truncates entries that are too long.  PostgreSQL
throws an error instead.  So we need to truncate the message field
before we try inserting into that column, to avoid errors from
very long messages in PostgreSQL (or MySQL configured to not allow
this).

Fixes bug #1217167

Change-Id: Ia959d64a8bc162a33ba818f1006e9779207b4796
Merge "Limit instance fault messages to 255 characters"