Handle removed files in tools/validate.py

Previously if a file was removed, no checks were done that the file
was not referenced anywhere.

Now you get e.g.:
Checking for removed files
  Removed file: doc/src/docbkx/common/section_keystone_cli_users_tenants_roles.xml
  Removed file: doc/src/docbkx/openstack-user-admin/src/section_cli_set_block_storage_quotas.xml
  Removed file: doc/src/docbkx/openstack-user-admin/src/section_cli_set_compute_quotas.xml
  Removed file: doc/src/docbkx/openstack-user-admin/src/section_keystone_cli_manage_projects_users.xml
  Removed file: doc/src/docbkx/openstack-user/src/section_cli_help.xml
  Removed file: doc/src/docbkx/openstack-user/src/section_cli_install.xml
  Removed file: doc/src/docbkx/openstack-user/src/section_cli_openrc.xml
  Removed file: doc/src/docbkx/openstack-user/src/section_cli_overview.xml
  Removed file: doc/src/docbkx/openstack-user/src/section_cli_version.xml
  File section_compute-configure-quotas.xml has an xi:include on deleted file ../openstack-user-admin/src/section_cli_set_compute_quotas.xml

And validate.py exists with error code to mark this as failure.

Also, limit number of parallel build jobs to 4 to reduce Jenkins load,
hope we see less org.apache.fop.fonts.FontTriplet.
Related-Bug: # 1221721

Change-Id: I3e47217958840f61347f71b159414928f266d6d5
Merge "Handle removed files in tools/validate.py"