Add missing tests for db.instance_type_* methods
Add missing tests for db.instance_type_* methods

There was no tests in test_db_api for instance_type_* methods.

Add tests to ensure that all works.
Also these tests allow us to refactor work with session and
to add missing unique constraints.

Fix bug in instance_type_get_all, we shouldn't remove
is_public from input filters dictionary.

blueprint db-api-tests

Change-Id: Ia02d73e6ffd964d6edb605039eb95e9f90ff0482

n0ano/ganttclient