Add missing test for db.instance_type_destroy method

This test checks that if there is no such instance_type
then is raised exception.InstanceTypeNotFound

blueprint db-api-tests

Change-Id: I0a0db7aaefe6ac781c520e3b90958800db8fd52b
Move db.instance_type_extra_specs_* to db.instance_type_* methods

All instance_type_* methods should be one after another

blueprint db-cleanup

Change-Id: Icc69c45dbcb3dcea5f81266a11f726b5b3215e27
Merge "Add missing test for db.instance_type_destroy method"
Merge "Move db.instance_type_extra_specs_* to db.instance_type_* methods"