Added host to UserIdentity so that the authorisation of adminstrators can be...

Added host to UserIdentity so that the authorisation of adminstrators can be based both on the user and the host from which they are communicating.

libs/client/UserIdentity.hpp

 #pragma once
 
 #include <stdint.h>
+#include <string>
 
 namespace cta {
 
@@ -19,6 +20,11 @@ struct UserIdentity {
    */
   uint32_t gid;
 
+  /**
+   * The host from which the user is communicating.
+   */
+  std::string host;
+
   /**
    * Constructor.
    *