merge home/foreign authority addresses

[snd]

currently in the config file home and foreign addresses of an authority are specified separately

parity-bridge/integration-tests/bridge_config.toml

Line 19 in 4520b67

account = "0x00bd138abd70e2f00903268f3db08f2d25677c9e"

parity-bridge/integration-tests/bridge_config.toml

Line 32 in 4520b67

account = "0x00bd138abd70e2f00903268f3db08f2d25677c9e"

imho there's no case where they should differ as there's only one address per authority in the authority list.

let's have only one config option for that. make configuration easier and removes one possible misconfiguration error