checkstyle/findbugs detected in validate phase

I changed the phase to validate for checkstyle+findbugs.

This makes PR Builder fail faster for checkstyle+findbugs problems. Also,this enables us to change PR builder configuration from **mvn clean install** to **mvn clean test** .

 mvn install is already causing problems if the machine  is the same where unmerged pr is built + installed and man.center+hazelcast-enterprise is built.