Attempting to enable a build agent that is down will remove it from the list

1. Add a single enabled build agent and enable distributed builds
2. Disable the build agent from the web interface
3. Shut down the build agent
4. Enable the build agent from the web interface

You will received the error:

Unable to ping build agent http://localhost:8181/continuum-buildagent/xmlrpc

It stays on the same page, but if you subsequently go to the build agent page, you'll find that the agent has been removed from the list