Use SpotBugs 3.1.2, and NP_NONNULL_PARAM_VIOLATION

Use SpotBugs 3.1.2, and NP_NONNULL_PARAM_VIOLATION

This patch upgrades the SpotBugs plugin to 3.1.3 and the engine to
3.1.2. The changelog is
https://github.com/spotbugs/spotbugs/blob/3.1.2/CHANGELOG.md

SpotBugs 3.1.2 supports the nullable annotations in Checker Framework
(see
https://github.com/spotbugs/spotbugs/commit/dd436ff56abf0273a080ad228fcf5abd8ee3a097
for the patch), so we can re-enable NP_NONNULL_PARAM_VIOLATION when
using SpotBugs. This involves using a separate configuration, so this
patch splits everything out, including the report directory for
projects which run both FindBugs and SpotBugs.

Change-Id: I8cb110f12d79414d476b7a6c525412f3b7e2c188
Signed-off-by: Stephen Kitt <skitt@redhat.com>