exchange: try all autodiscover entrypoints

Source: http://msdn.microsoft.com/en-us/library/office/jj900169%28v=exchg.150%29.aspx
The Exchange's entry point for email based autodiscover could be one of the following:

 * "https://" + domain + "/autodiscover/autodiscover" + fileExtension
 * "https://autodiscover." + domain + "/autodiscover/autodiscover" + fileExtension

Actually only the first one is tested, so setup could fail to resolve exchange configuration
if the server response to the second entry point.

Change-Id: Ieb4bce33a54997a910a4d6d06c172ebef132ec7c
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>