Read system properties from product.conf

Use bin/product.conf as a source of system properties, thus allowing layered products to set standard properties.

These properties should take lower precedence than those passed via other means (command line, -P, xml config file.)

As an aspect of this, take -Djboss.server.default.config=standalone.xml, -Djboss.domain.default.config=domain.xml and -Djboss.host.default.config=host.xml should be removed from standalone.conf/domain.conf. This will allow them to be set in product.conf by layered products that want a different default config file without having the standalone/domain.conf values taking precedence.