Refactor work with db.instance_type_* methods

Remove session parameter from public methods:
  *) instace_type_get
  *) instance_type_get_by_name
  *) instance_type_get_by_flavor_id

instance_type_destroy performance optimization:
  Get instance_type_ref without unnecessary extra specs

instance_type_access_get_by_flavor_id performance optimization:
was:
1 query get instance_type with joined extra_specs
1 query lazy load instance_type_projects
now:
1 query get instance_type id
1 query get all instance_type_projects that correspond to id

instance_type_access_add performance optimization:
was:
1 query get instance_type with extraspecs
1 query add instance_type_access
now:
1 get only instance_type id
1 query add instance_type_access

instance_type_access_remove performance optimization:
Same as in instance_type_access_add

blueprint db-session-cleanup

Change-Id: I6d1456289a97c46d7e8c9345474396c08e053970
Merge "Refactor work with db.instance_type_* methods"