Delegate code style validation to computers, they're better than human
at it :)
This commit adds a new `tools/validate-indentation.sh` script that
errors when it finds an indented Dockerfile instruction, and enables
the validation in the pep8 check.
Also adjust the logic of `tools/validate-all-dockerfiles.sh` so that it
doesn't stop on first errors but instead shows all of them.
Fixes the remaining indentations in Dockerfiles.
Change-Id: I53c0d38304cb4f6d64a5dfab67f70d69b3eae587