Training guide: Fix diagrams for PDF output

A lot of images are very large in PDF output, follow our documented
conventions to add contentwidth="7in" everywhere.
The Training Guides use landscape mode - thus we use "7in" instead
of the usual "6in".

Change-Id: I74052b864f65ed2a1199e5008a62b9a95cac9968
Closes-Bug: #1294054
Merge "Training guide: Fix diagrams for PDF output"