Based on work by Romain Courteaud <romain@nexedi.com>.

This is required for squid

Also present on others branches.

 because who setups using cluster mode should be able to run tests also :)

This change is required to expose openssl to the instance during the
Unit Tests.

For unknown reason, using disk cache does not support vary.
refresh-pattern has to be configured to use state-if-error.

Expose openssl binary to zope.

Yet another stupid commit.
Do not push local path in public repository.

Add the "http-cache": True parameter in the backend json definition to activate.

It seems the only tool supporting http://tools.ietf.org/html/rfc5861