Adjust validate.py for flattened directory structure

With this change validate.py validates and builds again the manuals.

I disabled validation of basic-install-pom.xml as it's a pom.xml
file.
Currently the basic-install guide is not build at all.

Change-Id: I9cabab5e03266ac7d5d4452ddbf97ea6d9dcdf3a
Merge "Adjust validate.py for flattened directory structure"