Implement all methods of Map in AppConfig

[yanchevsky]

These methods can be useful for testing.

    @Override
    public String put(String key, String value) {
        throw new UnsupportedOperationException("Operation not supported, not a real map");
    }

    @Override
    public String remove(Object key) {
        throw new UnsupportedOperationException("Operation not supported, not a real map");
    }

    @Override
    public void putAll(Map<? extends String, ? extends String> m) {
        throw new UnsupportedOperationException("Operation not supported, not a real map");
    }

    @Override
    public void clear() {
        throw new UnsupportedOperationException("Operation not supported, not a real map");
    }

    @Override
    public Set<String> keySet() {
        throw new UnsupportedOperationException("Operation not supported, not a real map");
    }

    @Override
    public Collection<String> values() {
        throw new UnsupportedOperationException("Operation not supported, not a real map");
    }

    @Override
    public Set<Entry<String, String>> entrySet() {
        throw new UnsupportedOperationException("Operation not supported, not a real map");
    }