Modernize ATCs script to Gerrit 2.8, programs.yaml

* tools/atc/README: Simplify instructions to use mysqlclient
tab-separated-values stdout, and remove obsolete manual steps which
have been automated.

* tools/atc/email-stats.sh: The functionality this shell script
previously provided has now been translated to Python and folded
into the inner script.

* tools/atc/email_stats.py: Implement the old email-stats.sh outer
loop, make it possible to run in a virtualenv, and make previously
hard-coded constants into command-line parameters with sane and
dynamically-determined defaults. Update field order and contents to
cope with differences in Gerrit 2.8, use a proper YAML parser on the
programs.yaml which has also changed format since the previous run,
and adjust for the improved extra-atcs file format as well.

Change-Id: I2deef2e6766ff998190e66737e7cec9cdcd5459c
Merge "Modernize ATCs script to Gerrit 2.8, programs.yaml"