MT fixes to provide initial support for tenant-specific guests

- explicit guest access is required, such as "guest@tenant1" (note: implicit/anonymous guest access can only login to the default domain)
- also fixes issue with "Show All" users, when logged in as a tenant admin

git-svn-id: https://svn.alfresco.com/repos/alfresco-enterprise/alfresco/HEAD/root@7748 c4b6b30b-aa2e-2d43-bbcb-ca4b014f7261

source/java/org/alfresco/repo/tenant/SingleTServiceImpl.java

@@ -128,14 +128,19 @@ public class SingleTServiceImpl implements TenantService
         return false;
     }
     
+    public String getUserDomain(String username)
+    {
+        return DEFAULT_DOMAIN;
+    }
+    
     public String getCurrentUserDomain()
     {
-        return "";
+        return DEFAULT_DOMAIN;
     }
     
     public String getDomain(String name)
     {
-        return "";
+        return DEFAULT_DOMAIN;
     }
     
     public String getDomainUser(String baseUsername, String tenantDomain)