Attempt to build all books

Modifies tools/validate.py to run "mvn clean generate-sources"
in every directory that has a pom.xml file.

The script will fail with an error if any of the books fail to build.
It will terminate on the first failure.

Fixes bug 1031374

For now, we aren't gating on openstack-training

Change-Id: Ic0136e4245e8e4bfa2e790574e3ec82c944c4b02
Merge "Attempt to build all books"