Support for two way/mutual authentication for http proxy upstream 

ngx_http_proxy module support one way authentication using trusted certificate only. If upstream server requires mutual authentication,  it fails.  I have added support for mutual authentication using certificate and key.

I have validated against Oracle WebLogic11g server.

The logic is as below.
<pre>
if proxy_ssl_trusted_certificate  is configured and (proxy_ssl_client_certificate or proxy_ssl_client_certificate_key) configured
   it logs warning for proxy_ssl_client_certificate or proxy_ssl_client_certificate_key will be ignored.
 
if   proxy_ssl_trusted_certificate is configured then 
    it use ssl_trusted_certificate for authentication
else if proxy_ssl_client_certificate  and proxy_ssl_client_certificate_key configured,
   it uses both to do two way authentication
else 
   logs error as either proxy_ssl_trusted_certificate  or (proxy_ssl_client_certificate and proxy_ssl_client_certificate_key) required.
</pre>

Added following two new config parameters:
<pre>
proxy_ssl_client_certificate  cert.pem;
proxy_ssl_client_certificate_key cert.key;
</pre>