engine: rename gwt-extension.

- Renamed gwt-extension.jar to ovirt-gwt-extension.jar

Change-Id: Ic53dbae3a56e0a3451c295e05f358a8cc782df4e
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1013752
Signed-off-by: Alexander Wels <awels@redhat.com>