Secure get_creator implementation

[g2flyer]

Description
Currently we just get the creator-name as-is from the untrusted part. To make it secure, we will have to pass the signed-proproposal to ecc which then will have to verify it and extract the creator info. As this also involves MSP validation and alike which tlcc has to do (see e.g., issue #125), this is probably best done via a RPC to TLCC.

[g2flyer]

tabled until we have trusted ledger ...

[mbrandenburger]

This is partially addressed with #637. We ensure that the creator identity returned by the function is indeed backed with the data from the signed transaction proposal. However, still, without the trusted ledger we cannot check the returned identity with the MSP of the channel.