Fall back to HTTP on right condition when HttpHostConnectException

[chanseokoh]

Fixes #767.

I think it makes sense to me to fall back to HTTP only when certain conditions are met: #767 (comment)

Does it make sense?

[coollog]

Looks good - although I'm wondering if there will be many repeated warning messages for someone trying to use an HTTP registry.

[chanseokoh]

Looks good - although I'm wondering if there will be many repeated warning messages for someone trying to use an HTTP registry.

That's a valid point. We could add a counter somewhere to print only a limited number of repeated warnings. But I don't think I'd like to add that feature now. I'd think it's a small price to pay for using insecure servers. However, I'll file an issue so that we can discuss.