Introduce IcingaApplication#environment

Introduce IcingaApplication#environment

Precedence as follows:

- DEnvironment=...
- const Environment = ...
- object IcingaApplication "app" { environment = "..." }

The wrapped script constant handling is required
since we cannot directly link from libremote (SNI handling)
to libicinga where the object resides. Instead we'll
use the Application class helpers for hiding the ScriptGlobal
calls.