Each of the TraitText, TraintInt, TraitFloat
and TraitDatetime have key column which has
its type defined incorrectly as Integer. The
migrate script correctly defines the type as
String(255).
This fix updates the sqlalchemy Trait* models
to define key as String(255).
Closes-Bug: 1449752
Change-Id: I5ac2e9b811e3baa53e329732233cd0011a4d25b9