librenms: another round of misc fixes

- Add a project name, title & logo
- Switch RRD storage to /srv/, as there's plenty of (LVM) space there.
- Enable mod_rewrite, since it's used by .htaccess
- Switch docroot to /html, as originally intended.
- Change Nagios check to mention "LibreNMS HTTP" in the description, to
  make clear which service is broken.
- Remove now virtual php5-json; add php-net-ipv4/6
- Fix a typo in one of the crontabs
- Fix config.php to not override LibreNMS' default settings

Change-Id: I80170aa558ff4d132c99582f4106731d980aae29