For connections that will attempt to use an HTTPS proxy with an HTTPS
destination, we'll use the TLS in TLS support provided by SSL Transport.

HTTPS proxy and HTTP destinations will continue using a single TLS
session as expected.

We'll still support the use of forwarding for HTTPS destinations with
HTTPS proxies as long as the "use_forwarding_for_https" parameter is
provided.

Signed-off-by: Jorge Lopez Silva <jalopezsilva@gmail.com>

- Make the toggle for absolute URI forwarding public and adjust
  documentation.
- Adjust documentation in util/proxy.py for connection_requires_http_tunnel.
- Only set the proxy_pass if the pass was provided.
- Adjust unsupported ssl.SSLContext check.
- Remove 'server_hostname' parameter on platforms without SNI support.
- Better handling and passing of destination scheme between
  connectionpool and connection.

- Mistake in previous commit.

- Improve documentation syntax.
- Use Python 2 instead of py2.
- Remove support for configuring through environment variables.
- Nit and suggestions on util.ssl_.py
- Improvements on validation and error messages for not supported
  SSLContext.

- Move to staticmethod instead of classmethod, also make it private.
- Error out if SSLTransport isn't available and we need to do
  TLS-in-TLS.
- Avoid modifying the signature of prepare_proxy.

- We no longer need to pass the upstream destination.

- Rename 'generate_proxy_ssl_context' to 'create_proxy_ssl_context'
- Remove unnecessary method to set the 'tls_in_tls_required' attribute.
- Improve handling of default ssl_context.
- Improve warning message for Python2.
- Fix missing assert on secure transport.
- Other test renames.