optional_plugins.loader_yaml: Allow name_prefix to be list

Allowing name_prefix to be list allows one to specified shared parts of
the name_prefix inherited from upper-node:

    !mux
    functional: !mux
        name_prefix: ["functional-"]
        rpm:
            name_prefix: ["rpm"]
        git:
            name_prefix: ["git"]

which results in: "functional-rpm" and "functional-git".
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

optional_plugins/loader_yaml/avocado_loader_yaml/__init__.py

@@ -114,6 +114,8 @@ class YamlTestsuiteLoader(loader.TestLoader):
                 test_loader.get_full_decorator_mapping())
             name_prefix = params.get("mux_suite_test_name_prefix")
             if _tests:
+                if isinstance(name_prefix, list):
+                    name_prefix = "".join(name_prefix)
                 for tst in _tests:
                     if name_prefix:
                         tst[1]["name"] = name_prefix + tst[1]["name"]