As a user
I want to set purge_type with environmental variable
So that I can configure it easier on Travis

In 2.5 it was possible to set purge_type using HTTPCACHE_PURGE_TYPE variable (https://github.com/ezsystems/ezplatform/blob/2.5/app/config/env/generic.php#L59). This made it easy to setup tests on Travis for Varnish.

Right now the value is hardcoded - https://github.com/ezsystems/ezplatform/blob/master/config/packages/ezplatform.yaml#L26 and cannot be easily changed on CI