packaging/configuration: get rid of _ENV

We should use _SERVER for our settings as it works fine on CLI and
for web servers.

refs #6400
packaging/config: rename ICINGA_APPDIR

Renamed ICINGA_APPDIR to ICINGAWEB_APPDIR for consistency. This way
it fits ICINGAWEB_CONFIGDIR.

refs #6400
packaging/config: allow to override APPDIR

It is now possible to either define ICINGAWEB_APPDIR in an Environment
variable or to define it as a PHP constant before bootstrapping.

fixes #6400
CLI/monitoring/list: fix formatting-related error

Small fix, this got broken by language-related changes in a66a1a47.