build: always clean maven output before build
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 <2e3ceb4d37adc6ae3ccc00d3646e0e0a036ace03@redhat.com>

halober/ovirt-engine,zerodengxinchao/ovirt-engine,eayun/ovirt-engine,walteryang47/ovirt-engine,yingyun001/ovirt-engine,OpenUniversity/ovirt-engine,walteryang47/ovirt-engine,zerodengxinchao/ovirt-engine,yingyun001/ovirt-engine,eayun/ovirt-engine,yingyun001/ovirt-engine,yapengsong/ovirt-engine,OpenUniversity/ovirt-engine,yingyun001/ovirt-engine,eayun/ovirt-engine,yapengsong/ovirt-engine,walteryang47/ovirt-engine,eayun/ovirt-engine,walteryang47/ovirt-engine,halober/ovirt-engine,OpenUniversity/ovirt-engine,eayun/ovirt-engine,walteryang47/ovirt-engine,halober/ovirt-engine,halober/ovirt-engine,yapengsong/ovirt-engine,zerodengxinchao/ovirt-engine,zerodengxinchao/ovirt-engine,OpenUniversity/ovirt-engine,zerodengxinchao/ovirt-engine,yapengsong/ovirt-engine,yapengsong/ovirt-engine,OpenUniversity/ovirt-engine,yingyun001/ovirt-engine