Come up with a better way to access the network passphrase in the wallet backend

[gouthamp-stellar]

Right now the network passphrase is being passed around to all the services that need them. Instead a cleaner way to implement this may be to have a helper function that simply returns the network passphrase so these services can use it

[JakeUrban]

The wallet backend doesn't index multiple networks (per instance) right? If so the network passphrase should be a config file, and I figured there would be a config object with all the config variables that different services can access.