[EAP7] There is no environment variable to configure the initial metaspace size

GC_MAX_METASPACE_SIZE is an environment variable available on jboss.container.java.jvm.api eap-module that allows setting the maximum metaspace size, however, there isn't any variable that allows setting the initial size.

The alternative is to use JAVA_OPTS to configure it. Since there is one variable to the maximum values, looks reasonable to have one to the initial size.