admin: wrap data_admin.py lint in a tox env

To get the 'data_admin.py lint' triggered in Jenkins, the easiest
solution for me is to have it wrapped in a tox env.  We have all the
machinery available to trivially add the job.

Tox uses virtualenv which provides a void environement.  Since
data_admin.py depends on a yaml module, define PyYAML in the rather
standard 'requirements.txt' file.

Example usage:

$ tox -e data_admin_lint
data_admin_lint runtests: commands[0] | python data_admin.py lint
__________________ summary ___________________
  data_admin_lint: commands succeeded
  congratulations :)
$

Change-Id: Ibe5b9ea99425137dec6cce1f86b86b63410f6ddb
Merge "admin: wrap data_admin.py lint in a tox env" into production