feature: env flag to prevent account-level default trunk routing

[davehorton]

In some larger multi-tenant deployments it may be desirable to disallow API requests to set a default application at the carrier level. As an example, it is possible that many accounts will be using Twilio as a carrier, and in the first place we should not allow two of them to default route all calls from twilio. Currently, the first one that selects to default route twilio would "win" but as soon as a second set twilio as a default route calls would stop working for the first. It is not clear that we would want anyone in this scenario to be able to default route twilio; ie force everyone using twilio to enter a phone number and route based on that.

On the other hand, a customer entering their own SBC for a sip trunk we would want to be able to default route...