Add support for HTTP proxy

The plugin does not work when the Sonarqube instance needs to use a HTTP proxy to access the OpenId server. This patch configure the openid4java library to use the proxy configured in the sonarqube properties (`http.proxyHost`, `http.proxyPort`, `http.proxyUser`, `http.proxyPassword`). It also add a `sonar.openid.useProxy` property which can be set to use or not the proxy configuration.