FORGE-1953: Add command to get and set JVM system properties

This PR is intended to implement the feature request https://issues.jboss.org/browse/FORGE-1953.

The idea is to add a new command to get/set system properties.
System.setProperty may return null if the property does not exist (see javadoc).
Is this ok? we should make it behave like system-property-get with an unknown property that will show nothing instead of null (as suggested)