QNetworkProxyFactory: check all the proxy environment variables

QNetworkProxyFactory used to check only for the http_proxy environment
variable in systemProxyForQuery. This patch makes it look as well in
https_proxy, ftp_proxy and all_proxy. http_proxy is still used as a
fallback value.

Change-Id: I7934af70d191cd17dbce3b3789260ae1a8332986
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Shane Kearns <shane.kearns@accenture.com>