Mirror-registry: plain-text default csrf secret key

A flaw was found when using mirror-registry to install Quay. It uses a default secret, which is stored in plain-text format in one of the configuration template files. This issue may lead to all instances of Quay deployed using mirror-registry to have the same secret key. This flaw allows a malicious actor to craft session cookies and as a consequence, it may lead to gaining access to the affected Quay instance.