Database: fix get_property_forced() and get_device_property_list with StdStringVector

Database.get_property_forced() and Database.get_device_property_list() with a StdStringVector container both raised a TypeError: the bindings required the trailing DbServerCache argument of the C++ calls, but DbServerCache is not bound (it is only forward declared in the public cppTango headers), so the cache pointer can never be anything but null and tango/db.py cannot pass it. Both are now bound through a lambda that leaves the argument out of the Python signature, and the StdStringVector branch of get_device_property_list returns the filled container instead of None. Tests were added for both methods.