double key problem

[SurfingNerd]

Currently it is possible to define a node that has a different signing key for the service-transactions than it has for hbbft-messages.

example location of the key files:

hbbft-key:      /data/network/key
transactions: /data/keys/DPoSChain/hbbft_validator_key.json

That leads to communication Issues as described here: #62

The HBBFT implementation expects that this two keys are the same.
In order to avoid this problem we should make it a single point of configuration,
were only one key has to be defined.

If that's to hard to implement:
Only make a check on startup if both keys are the same.