Feature: Adds support to configure Director Self-Service config string

[LordHepipud]

This new feature allows to configure the Icinga Director Self-Service call during registration, by modifing the JSON-String. This will only apply to the first time registration of a host and is not modified again, unless the host and director keys for this specific host object are removed.

This works directly inside the IMC installation wizard as well as for the exported installation command or file.

By doing so, you can manually define the IP-Address with address key in your JSON as well as the display name of an object with display_name. Example:

{ "address": "127.0.0.1", "display_name": "Example Host" }

The following constants are available inside the configuration:

$ifw.hostaddress$: Will use the default IP-Address automatically fetched from the system
$ifw.hostname$: The current hostname of the host, without FQDN and not modiying upper/lowercase
$ifw.hostfqdn$: The current FQDN of the host, without case modification

For lower and upper case modification, you can use .tolower or .toupper for the hostname and FQDN, example:

{ "address": "127.0.0.1", "display_name": "$ifw.hostname.tolower$" }

Fixes #292
Fixes #544