Improve documentation and debug logging for issue with non-reachable identity provider due to network proxy

[tjuerge]

If a network proxy is used with SonarQube (via http[s].proxy[Host|Port] properties in the sonar.properties) and the host name of the identity provider is not resolvable by this proxy then the IdP's host name must be excluded from being resolved by the proxy. This is done by defining the property http.nonProxyHosts in the sonar.properties.

Otherwise the plugin won't be able to send the token request to the IdP.