Returning NexusApp level to INFO level.

I traced back, NexusApp was set to ERROR level
even in SVN (history way back), and probably case
(at least according to comments in log4j.props file
from then) points that the reason was a problem we
recorded later (and fixed) as NEXUS-3442.

This fix went out with Nexus 2.3, but, the "muted"
NexusApp is still at level ERROR. Hence, due to this,
we loose precious messages like those emitted by this pull
https://github.com/sonatype/nexus/pull/745

It emits WARNs while log contains nothing as NexusApp is
at ERROR level.

Putting NexusApp back to INFO and we will fix any unneeded
logging anyway.