Software deployments have FK constraints on software configs.
This change ensures the DBReferenceError caused by the constraints is
properly caught. With this change now heat returns 400 Bad Response,
instead of 500 Internal Server Error when a user tries to create
a software deployment with a non-existing software config.
Also, the stack_user_project_id field is defined as 64-chars-long
string in DB model, so we should ensure that the input value is shorter
than 65 chars. Otherwise it also results in DB error.
Story: 2010001
Task: 45098
Change-Id: I03274dc0cffa226140eb720458cce81e8b5ce187
7860e7acbc