change the backupninja::client::key definition to use the $user variable

change the backupninja::client::key definition to use the $user variable
change the $host variable to no longer use the $user variable, that way
multiple rdiff's can be instantiated for a particular host, with different
name variables
in order to do that, the class also has to have the $installkeys variable that
can be passed to backupninja::client::key so that the second rdiff instantiation would
set installkeys => false, otherwise there will be an attempt to duplicate the resource