GlusterFS stack doesn't allow to change fs.defaultFS

When you create Hadoop cluster based on GlusterFS stack, there are properties
that are defined in core-site config file, but can't be changed via Ambari web
interface.

The config tab of GLUSTERFS service doesn't provide any way to edit properties
fs.defaultFS and fs.default.name (which is deprecated alias of the
previous one) from core-site.xml config file.