Allow setting encryption key before joining the group

[ayurchen]

Otherwise there is a plaintext leak in the very beginning of the writeset stream.

Currently src/server_state.cpp has

wsrep::server_state::set_encryption_key(std::vector<unsigned char>& key)
{
    encryption_key_ = key;
    if (state_ != s_disconnected)
    {
      ...

i.e. setting the key is skipped if provider is not connected. This is clearly a mistake as we need the key to be set before provider starts to receive and cache any data.