DocBuilder: fix installing twice to different destinations

Also, allow specifying top project directly in DocBuilder ctor.

Change-Id: I1acce26a5b1b88d9bc55b00541a2d308295ec684
Reviewed-on: http://gerrit.aldebaran.lan/33586
Tested-by: gerrit
Reviewed-by: vbarbaresi <vbarbaresi@aldebaran-robotics.com>
Reviewed-by: dmerejkowsky <dmerejkowsky@aldebaran-robotics.com>