Allow spaces in java home. (apache#11407)

Quote the $java_exec var in examples/bin/verify-java to support spaces in DRUID_JAVA_HOME/JAVA_HOME. At present, the steps before and after the version check properly quote the path, but the version check spuriously fails when pointing to a Java 8 install that has a space in its path.

examples/bin/verify-java

@@ -59,7 +59,7 @@ if ($java_bin_dir eq "") {
   fail_check()
 }
 my $java_exec = "${java_bin_dir}/java";
-my $java_version = qx[$java_exec -version 2>&1];
+my $java_version = qx["$java_exec" -version 2>&1];
 if ($?) {
   fail_check();
 }