New feature: allow overriding cookie_domain from system env var

We have the following use case: we want to share the authentication cookie __ac between several domains. We use cookie_domain to achieve this, but it creates several problems: authentication doesn't work anymore through tunnels and also developers need to always make changes in the database when working on a new data.fs copy.

To overcome this we have introduced the possibility to override the cookie_domain based on an environment variable called PLONE_COOKIE_DOMAIN. This can be set directly from the command line or from buildout configuration.