Specialize native faucet configuration

[Mirko-von-Leipzig]

Simplify the native faucet specification in the genesis configuration.

#1122 added basic fee support to the node. As part of this it added support for specifying the native asset in the genesis configuration file. At the moment this asset's faucet is specified as a conventional basic fungible faucet, however the native faucet must

• use public storage, and
• have exactly one native faucet declared

A better configuration would be specializing the native faucet as suggested here, which would enforce that this native faucet exists and prevent storage from being configured.

In addition, this should also be carried through from the genesis config file to the actual state creation i.e. it should take in a native faucet in addition to the extra, arbitrary accounts.

[bobbinth]

@drahnr this is closed by #1157, right?