Deploy a legacy pool for PHP 5.6 applications in webserver-legacy role

Summary:
PHP 5.6

The unit was fairly ready to be used for multiple instances, but needed
some improvements:

• don't hardcore 'prod' (the name of the production pool)
• when a site is moved from one pool to another, there is a need to remove old files in previous pool (cleanup.sls does that)

Pure Python state formula dumper script now supports os grain to be able
to dump cleanup.sls too.

Ref T1469

Test Plan:
https://www.espace-win.org/phpinfo.php shows an expected
configuration, with php.ini loaded.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1469

Differential Revision: https://devcentral.nasqueron.org/D1947