Add new option --build-file-exception to openstack-doc-test

This allows to ignore files just for syntax and niceness checks
but still use them for calculation of dependencies.

This allows us to handle the training-guide where st-training-guides.xml
should be black-listed for syntax checks but needs to be used for
dependency calculation in delete and build checks.

Change-Id: Ib183e635b623218083447e85fe8a6b0cb278a9e6
Merge "Add new option --build-file-exception to openstack-doc-test"