PyUtil::_class_factory: Pass custom prefixes

cppTango added logic to try custom prefixes of shared libraries with device servers. The idea was that this would allow shared libraries like lib.so to be picked up when .so was requested. But that is broken.

Workaround that by passing in a custom library prefix. With this fix and a fixed TangoTest naming 2 the test test_restart_server_command_cpp_and_py passes on debian. On debian ldconfig does not pickup libraries without "lib" prefix.

Close #649 (closed)