Allow static definition of the product module slot

The way a WildFly Core based distribution is able to control metadata such as the reported product name and version is by packaging an org.jboss.as.product:xxx module and then including a bin/product.conf file with a properties file that includes property slot=xxx. The ProductConfig class looks for bin/product.conf and reads it from the filesystem.

This does not work for WildFly Swarm, which is not based on an expanded filesystem. There is no bin/product.conf file, not even a bin dir.

This RFE is to have ProductConfig allow the slot name to be directly passed in, skipping the property file reading, with SelfContainerContainer (used by swarm) passing in a static value of "main". So swarm can package a org.jboss.as.product:main module and use it to control the product settings.