Support for managing external subnet routing announcements #9583
Description
As part of the external subnet work (RFD 599), we would like to provide operators the choice to opt-in to announcing these prefixes through BGP. This issue covers forwarding the prefixes for external subnets that are attached to instances over to Maghemite, where it will announce them via BGP. We also need to withdraw those announcements when the subnet is detached from an instance.
This is all opt-in, so an operator needs the top-level control to turn this all on or off.