Add OpenID SSO support.

Configuring OPENID_SSO in gerrit.config will allow the admin
to specify an SSO entry point URL so that users clicking on
"Sign In" are sent directly to that URL.

Change-Id: I361b7b74a6f34a199578620540be31d1df0c5fef

gerrit-httpd/src/main/java/com/google/gerrit/httpd/GerritConfigProvider.java

@@ -90,6 +90,10 @@ class GerritConfigProvider implements Provider<GerritConfig> {
         config.setAllowedOpenIDs(authConfig.getAllowedOpenIDs());
         break;
 
+      case OPENID_SSO:
+        config.setOpenIdSsoUrl(authConfig.getOpenIdSsoUrl());
+        break;
+
       case LDAP:
       case LDAP_BIND:
         config.setRegisterUrl(cfg.getString("auth", null, "registerurl"));

gerrit-httpd/src/main/java/com/google/gerrit/httpd/WebModule.java

@@ -108,6 +108,7 @@ public class WebModule extends FactoryModule {
         break;
 
       case OPENID:
+      case OPENID_SSO:
         // OpenID support is bound in WebAppInitializer and Daemon.
       case CUSTOM_EXTENSION:
         break;