Add sshd-keygen.target to abstract key creation from sshd.service and sshd@.service (#1325535)
    
     * PartOf  is needed to trigger  sshd-keygen  checks for  sshd.service  restarts
     * sshd-keygen.target  makes a level of abstraction to eliminate dupplicate
       dependencies on both  sshd  and  sshd@  services