Review request: JMC-6113: enable gpg-sign only for deploy phase

[Guru]

Please review the fix for https://bugs.openjdk.java.net/browse/JMC-6113 <https://bugs.openjdk.java.net/browse/JMC-6113> 
Web rev : http://cr.openjdk.java.net/~ghb/JMC-6113/webrev.0/ <http://cr.openjdk.java.net/~ghb/JMC-6113/webrev.0/>

Thanks,
Guru