NEXUS-6099: Added 2.8 model for Nx core

So far, it's a noop upgrade, merely version change
to align model and core version.

Hopefully, this is last Modello based config,
NEXUS-6099: LVO change

Aligned model version with core version,
using new features from core to manage configuration.
NEXUS-6099: Kenai Realm change

Aligned model version with core version,
using new features from core to manage configuration.
NEXUS-6099: LDAP OSS configuration aligned

Bumped version to align it with core, using new
core stuff to manage configuration.
NEXUS-6099: Bump system property

Missed this
NEXUS-6099: Handle IOEx in resources.
NEXUS-6099: Improve error reporting of ModelUtils
NEXUS-6099: Enforce model versions when saving model
NEXUS-6099: Translating NIO file not found to IO
NEXUS-6099: Adjusted UT
NEXUS-6099: Smaller improvements

Keeping modello writers and readers as we use them in thread safe
way. LDAP not keeping AppConfig instance as it does not need it
NEXUS-6099: Lax OSS LDAP XML versioning

It turns out, that OSS ldap.xml files were laxed
as version file is concerned, especially as there was
no model change since "1.0.0" (1.0.1 just added
an optional field).
NEXUS-6099: Inappropriate comment
NEXUS-6099: Kenai versioning lax
NEXUS-6099: Fixing IT

They were basically using invalid config files, as they
override method in superclass doing interpolation.
NEXUS-6099: fixed ordering in ITs

As config was first looked up, and then only was
the XML file put under the test.
NEXUS-6099: Making Kenai Realm config lazy again

As it's trivially simple: is read only, and it interferes
with Kenai ITs that boot nexus to be able to create kenai-role
that is already in eagerly loaded config (causing problems
and Kenai plugin realm being discarded).

This way, Kenai config is lazy again, that does not stirs
any water in production, but makes ITs work again.
NEXUS-6099: Using proper exception for this condition

As this ex was added later, still to be able to differentiate
between unreadable XML and the one without version.
NEXUS-6099: Making utils class a bit chattier
Merge remote-tracking branch 'origin/master' into NEXUS-6099-desupport-1x-plugins