Fix issues in validate_metadata.

Options:

• Use get_param function to simplify the logic (ex: if get_param(value) == "environment_context")
• Use any function to simplify the logic and determine if any of the metadata is environment_context (ex: has_env_context = any(get_param(value) == "environment_context" for value in metadata.values())
• Then if has_env_context is True return the error message if the parameter type of environment context is not True (there is already a check that all get_param calls in a Heat template have their corresponding parameter defined so there's no need to account for that in the error message)