[JENKINS-46288] - Fix ProxyConfiguration validation for NTLM authentication (#2984)

/**
     * Constructor.
     * @param userName The user name.  This should not include the domain to authenticate with.
     * For example: "user" is correct whereas "DOMAIN\\user" is not.
     * @param password The password.
     * @param host The host the authentication request is originating from.  Essentially, the
     * computer name for this machine.
     * @param domain The domain to authenticate within.
     */

(cherry picked from commit ccb3e4cd)