Refactor software config db model to use LongText

Reuse the types defined in heat.db.sqlalchemy.types in software config
db model and versions migration to make code clean and consistent.

Change-Id: Iae7d5ec34413aedf8a38a3e22d22cacf4e031c24
Add heat.sqlite in git ignore list

SQLite file "heat.sqlite" will be generated during test cases
running. So it should be in git ignore list.

Change-Id: I31217e3de69a016fb4daa7e353161d58c71df98b
Merge "Refactor software config db model to use LongText"
Merge "Add heat.sqlite in git ignore list"