ganeti.http: Implement SSL for HTTP client

Implementing SSL for the HTTP client required more work than I expected.
For correct error handling, quite a lot of code is needed. To avoid
code duplication, I moved a lot of the socket handling code into a
single function named _SocketOperation. It takes care of the polling
and error handling for OpenSSL. At the same time, the functions calling
it become a lot smaller.

This patch brings a few more changes:
- Error handling for socket.connect_ex (e.g. name resolution failures)
- Rename "ResponseError" exception class to "_HttpClientError" --
  it's an internal class anyway

Reviewed-by: iustinp