Fix method VolumeTypeList.get_all

Fix method VolumeTypeList.get_all

Function volume_types.get_all_types returns dict, then
VolumeTypeList.get_all passes it to base.obj_make_list, which, in turn,
passes it to oslo_versionedobjects.base.obj_make_list.

And this function iterates over 4th argument and calls
item_cls._from_db_object for each item. So if it will iterate over dict
from volume_types.get_all_types, it'll get strings instead of orm
objects.

By the way, VolumeTypeList.get_all is not used anywhere in cinder code.
I tried to use it in unit test for my new patch and found this problem.

Closes-Bug: #1505653

Change-Id: If789bc5bf224974de5c4e4b9a57c83ef51d1779b