PolyDriver::getImplementation returns dd->getImplementation

This change is due to propagate the getImplementation through nested
PolyDrivers untill it end in the real device driver.
The reason is that the view done on plugin devices were failing.
fix output of yarpdev --list for plugins

The default wrapper was not shown for plugins. This fix the issue
yarp plugin fix: The default wrapper is now instantiated using yarpdev command
from terminal.

A new variable is added in yarpPluginSetting to store the wrapper name and it
is used when calling the factory.