validate modified files and build affected books
validate modified files and build affected books

At the moment the validation script simply validates all
files and builds all books afterwards. This results in a
very long runtime (appr. 30 minutes at the moment).

With this patch only modified files are validated and only
affected books are built. A book is affected if one XML
file includes a modified file. If no book is directly
affected all books are built.

In this way, resources can be saved and validation is
performed much faster.

Change-Id: I17dcf119c0bafc4656c9f267f0d3f5ae884ea76e

savinash47/openstack-doc-tools,savinash47/openstack-doc-tools,savinash47/openstack-doc-tools,openstack/openstack-doc-tools,openstack/openstack-doc-tools