Step four of the WFLY-18789 'epic'.

Allow an external caller to set a property that tells the testuite to use the channels available in the reactor when provisioning.

If set, when configuring testsuite provisioning by the wildfly-maven-plugin (see WFLY-18790), pass dynamically determine a set of channels to use and use the -Dwildfly.channels system property to pass those to the plugin execution.

This is similar to WFLY-18792, but is meant for cases where the caller doesn't want to explicitly specify channels, it wants to use what's known to the build.

See https://docs.wildfly.org/wildfly-maven-plugin/releases/4.2/dev-mojo.html#channels for more re the -Dwildfly.channels system property.