Allow selecting specific testbed node in addition to closest.

[a-thieme]

On the /connection/add page in the "NDN Testbed with NDNCert Bootstrapping" section, changing the "Closest Testbed Node" field does not change the testbed node you connect to.

This may be intentional, but it would be nice to have the ability to change this for debugging purposes.

[zjkmxy]

This is not intentional. It caused by a change that we always try to connect to the closest node. In future, maybe we should separate "connect to the closest node" and "connect to a specific node".