2.7.0

Changelog:
Benedikt Heine (11):
do not work around weird warning messages in feature-disable
validate config to log to stdout before starting supvervisor
use DEBIAN_FRONTEND only during installation
add special nagios plugins
hardcode graphite url, as ICINGA2_FEATURE_GRAPHITE_HOST is empty during initialisation
move position of args to use cache on change of buildargs
adapt lower layer for debian stretch
use variable repo name based on codename
update imagetag in readme
remove debian-sys-maint user
use mariadb packages