core: Fix findbugs to conform to mvn 3.0

In order to run mvn findbugs:findbugs,
the findbugs pluging configuration must exist at the
"build" section.
In addition, to avoid findbugs caching bug, ${basedir} should be specified as
the parent directory for the exclude filters file.

For more info, see here -
https://cwiki.apache.org/confluence/display/MAVEN/Maven+3.x+Compatibility+Notes#Maven3.xCompatibilityNotes-SiteandReporting

Change-Id: Id674955625a14c6e02a579fe2f132f64a8db9cc2
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>