Add description text for logging options.

global option to enable pagespeed with nginx - allows to enable pagespeed even if no webserver-config is selected

Added a server migration mode setting which will be used to prevent that LE tries to recreate certs during a server migration.

- made placeholders visible for apache, too
- added event raise at formdef loading (on_before_formdef and on_after_formdef)
- added possibility to put plugin files into module/lib/plugin.d/ directory

FS#3526 - Add option to disable BIND zone check (the intention for the feature request was more to hide log-messages and not to disable the named-checkzone-call)