build: always clean maven output before build

If snapshots (timestamp) are used then we get more than one artifact if
make clean is not run.

This confuses:
        X=`find "$(MAVEN_OUTPUT_DIR)" -name
	'engine-server-ear-$(POM_VERSION)*'.ear` && unzip -o "$$X" -d
	"$(DESTDIR)$(PKG_EAR_DIR)"

Change-Id: I6db059914f94bfab95cf444a0485a8f73a2719f3
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>