Explicitly set OpenJDK7 for builds

Now that we have openjdk7 and openjdk8 defined in Jenkins we should
explicitly configure the JDK used in the templates so that it is clear
which one is being used. This patch sets openjdk7 as the default builder
JDK.

Change-Id: Ibace9bea81e5cb153af75bd37d0b802d0145cd3f
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>