Add environment hooks for custom AS/router

[egourlao]

This pull requests adds environment hooks to support the use of custom Account Servers or Router URIs.

With these edits, the script can be used this way:

export ACCOUNT_SERVER_DOMAIN=canary.account.thethings.network
export ROUTER_MQTT_ADDRESS=canary.router.thethings.network:1882
curl https://some.domain.org/installer.sh | bash
# This setup will use the pre-configured AS and router configuration

[egourlao]

@kersing Any news about this pull request? :)

[kersing]

I am looking at another solution to achieve the same result. The installer is interactive so prompting for the information is more in-line with the existing code.

[Eelviny]

I'm going to vouch for @egourlao's method - something like this is a specialised setup, as 95% of people are just running something on TTN, and don't need to be presented with this option. I can handle setting environment variables for private instances.