Add flag to make clairctl serve a HTTP 200 OK on /

[jdel]

When pushing local images, because the server is only up for the small amount of time needed to serve the layers, it is nearly impossible to test connectivity from clair to clairctl with curl or telnet.

It would be good for troubleshooting purpose to have a flag (--listen ?) that can be passed to clairctl to make it serve a simple 200 OK on / for troubleshooting communication issues.