Fix wrong UniqueConstraint name

Table `sourseassoc` has a conflicted with
OpenStack naming convention UniqueConstraint name.
Each UniqueConstraint should be named as
`uniq_table_name0column_first0column_second`.

bp: ceilometer-db-sync-models-with-migrations

Change-Id: Ic35a9351add9dd3ce014c938505e268e131112e8
Merge "Fix wrong UniqueConstraint name"