Authentication: Add backend to handle external authentication

Drop external auth configuration from config.ini and move
implementation into a single backend provider named
'autologin'. This provider can strip realm names from
username with a custom regexp.

fixes #6081
Enable icinga snapshot repository and fetch latest icinga2 packages

Furthermore get rid of multiple 'exec' calls and use 'package' instead.
Additional refactoring for icinga 2 features.

fixes #6405
Import module 'conftool' for cli.

Refs #5929
Refactor fetching v1 attributes from parent template tree.

Refs #5929
Dump Icinga 2 objects, part 1.

Refs #5929
Add global assign/ignore where (Icinga 2).

Refs #5929
cli cfg: Add servicegroups and basic docs for Icinga 2 migration.

Refs #5929
cli cfg: Add usergroups for Icinga 2 migration.

Refs #5929
cli cfg: Host, Service, User Icinga 2 objects. (WIP)

Refs #5929
cli cfg: Add support for 'import' migration

refs #5929
cli cfg: First attempt of service->host{,group} relations

refs #5929
cli cfg: Convert address* and retry|check_interval

refs #5929
cli cfg: Apply services based on hosts and hostgroups

refs #5929
cli cfg: Make sure to pass config definitions to conversion hooks (WIP)

refs #5929
cli cfg: Rename 'parse' command to 'migrate'

Refs #5929
cli cfg: Add temporary debug output.
cli cfg: Convert runtime macros in command line

Refs #5929
cli cfg: Extract contact{,_groups} and convert to notifications (wip)

Refs #5929
cli cfg: Implement support for check command args and custom vars

refs #5929
cli cfg: disable debug dumps