metaelement.py: Provide constructor with default keyword arguments

    #    sandbox: Configuration specific to the sandbox environment

    #    first_pass: The element is to be loaded with first pass configuration (junction)

    #

    def __init__(self, project, name, kind, provenance, sources, config,

                 variables, environment, env_nocache, public, sandbox,

                 first_pass):

    def __init__(self, project, name, kind=None, provenance=None, sources=None, config=None,

                 variables=None, environment=None, env_nocache=None, public=None,

                 sandbox=None, first_pass=False):

        self.project = project

        self.name = name

        self.kind = kind

        self.provenance = provenance

        self.sources = sources

        self.config = config

        self.variables = variables

        self.environment = environment

        self.env_nocache = env_nocache

        self.public = public

        self.sandbox = sandbox

        self.config = config or {}

        self.variables = variables or {}

        self.environment = environment or {}

        self.env_nocache = env_nocache or []

        self.public = public or {}

        self.sandbox = sandbox or {}

        self.build_dependencies = []

        self.dependencies = []

        self.first_pass = first_pass