Fixed invalid default value for SESSION_DOMAIN

As from commit 522cbd4 the default value for
SESSION_DOMAIN is now set to "apc" if it's not supplied. This will lead to
CSRF token validation failures as described on
cachethq/cachet#2741 (comment)

With this commit the SESSION_DOMAIN will just be empty when not explicitly
configured.

Author: friek

entrypoint.sh

@@ -104,7 +104,7 @@ initialize_system() {
   CACHE_DRIVER=${CACHE_DRIVER:-apc}
 
   SESSION_DRIVER=${SESSION_DRIVER:-apc}
-  SESSION_DOMAIN=${SESSION_DOMAIN:-apc}
+  SESSION_DOMAIN=${SESSION_DOMAIN:-}
   SESSION_SECURE_COOKIE=${SESSION_SECURE_COOKIE:-}
 
   QUEUE_DRIVER=${QUEUE_DRIVER:-database}