Added a check to prevent that password protected ssl keys get saved. Thanks to Timo for finding the issue.

- 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

- fixed letsencrypt domain list

- added cron for letsencrypt renewal

- backported patches

see:
- http://git.ispconfig.org/panders/ispconfig3/commit/f7ec00b2f8ba3efc5bdeacef9c813f8a826ae3be#note_455

- http://en.wikipedia.org/wiki/SPDY

- Directive snippets can now be made available to clients; clients can select an available directive snippet for a website - the appropriate snippet will then be written to the vhost configuration file. This is useful for example on nginx where clients have no access to the Options tab of a website.

-Added a new option "Network Filesystem" under System > Server config > Web to toggle on / off the _netdev option for the bind mounts in /etc/fstab