tmf: Enable the finish button when default target project is selected

tmf: Enable the finish button when default target project is selected

The previous fix for the batch import "target project" page introduces
a default selection but the user still has to click on the list again
to enable the finish button.

Change-Id: I492cc1ff115d6208aa577a721f04d96ebd2a4e2c
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/19881
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>