Honor HTTP proxy settings on connection probing

Previously, when Rally probed for an Internet connection, it just opened
a plain socket connection. This is problematic when a user connects via
a proxy to the Internet because Rally will then conclude that there is
no working Internet connection at all.

With this commit, we respect the HTTP proxy setting on connection
probing too.

Closes #245

esrally/utils/net.py

@@ -1,7 +1,6 @@
 import logging
 import os
 import shutil
-import socket
 
 import certifi
 import urllib3
@@ -57,10 +56,16 @@ def retrieve_content_as_string(url):
 
 def has_internet_connection():
     try:
-        # We connect to Github anyway later on so we use that to avoid touching too much different remote endpoints
-        socket.create_connection(("www.github.com", 80))
-        return True
-    except OSError:
+        # We connect to Github anyway later on so we use that to avoid touching too much different remote endpoints.
+        probing_url = "https://github.com/"
+        logger.debug("Checking for internet connection against [%s]" % probing_url)
+        # We do a HTTP request here to respect the HTTP proxy setting. If we'd open a plain socket connection we circumvent the
+        # proxy and erroneously conclude we don't have an Internet connection.
+        response = __http().request("GET", probing_url)
+        status = response.status
+        logger.debug("Probing result is HTTP status [%s]" % str(status))
+        return status == 200
+    except BaseException:
         return False