An unconfigured build agent should not be enabled

I noticed that when I added a build agent that I had installed and started, (but not configured by adding the necessary items to buildagent-configuration.xml,) it responded to the ping and showed as 'enabled' in the master.

Without the master url it has no chance of actually working, so I think it shouldn't respond to the ping. Not being able to 'enable' it should cause the admin to go looking for the problem.

This might be fixed by a related issue, that an agent should only respond to its configured master. If there is no master configured, then it shouldn't be talking to anyone.