One account should be used for home and foreign networks

[akolotov]

As per discussion with the team which is doing security audit of the bridge we decided to have one account to sign transactions in home and foreign network (https://github.com/secureware/reports/blob/master/POA_Network-POA_Bridge_Security_Assessment.pdf).

So, consider to change handling of the bridge configuration file as per the following structure:

keystore = "/path/to/keystore"
account = "0x006e27b6a72e1f34c626762f3c4761547aff1421"
password = "password.txt"

[home]
rpc_host = "http://localhost"
rpc_port = 8545
required_confirmations = 0

[foreign]
rpc_host = "http://localhost"
rpc_port = 9545
required_confirmations = 0
...