Add artifact base target to nexus configuration

Add artifact base target to nexus configuration

Projects should be able to push to $groupId.$artifactId.$extension as
well well as anything under that path. Not allowing this means that our
projects always are having to define a broken pom that may look like:

<groupId>org.example.artifactId</groupId>
<artifactId>artifactId</artifactId>

Which means they always end up deploying a base artifact at
$groupId.$artifactId.$artifactId in many cases.

Change-Id: I11a2bcebfe07d39251d85f92ab783c1a2ed5288c
Signed-off-by: Andrew Grimberg <agrimberg@linuxfoundation.org>