Step three of the WFCORE-18789 'epic'.
Allow an external caller to set a property that declares the channels to use when testing.
If set, when configuring testsuite provisioning by the wildfly-maven-plugin (see
WFLY-18790), pass the -Dwildfly.channels system property to the plugin execution, with the value set to whatever the external caller provided.
This is similar to
WFLY-18793, but is meant for cases where the callers wants to use channels different from what may be available to the build (i.e. externally managed channels.)
See https://docs.wildfly.org/wildfly-maven-plugin/releases/4.2/dev-mojo.html#channels for more re the -Dwildfly.channels system property.