Fix output of tools/validate.py --force

Previously:
Validation of all files and build of all books will be forced.

Following files will be validated:
>>> doc/src/docbkx/basic-install/src/basic-install_compute-common.xml
>>> doc/src/docbkx/basic-install/src/basic-install_controller-dashboard.xml
>>> doc/src/docbkx/basic-install/src/basic-install_controller-keystone.xml
>>> doc/src/docbkx/basic-install/src/basic-install_controller-neutron.xml
>>> doc/src/docbkx/basic-install/src/basic-install_controller-nova.xml
...

But that information is wrong since all files will get validated. Now
you get:
Validation of all files and build of all books will be forced.

Validating all files
file admin-guide-cloud/ch_networking.xml:
...

Change-Id: If07544f532bd6861a23f52aa7fda044d0673cccc