Open Stack

I am trying to login into VM from host using ssh key pair instead of
password. I have created VM using keypair *khayamkey* and than tried to
login into vm using following command

ssh -l tux -i khayamkey.pem 10.3.24.56

where *tux* is username for VM, but I got following error

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!Someone could be
eavesdropping on you right now (man-in-the-middle attack)!It is also
possible that a host key has just been changed.The fingerprint for the
RSA key sent by the remote host
is52:5c:47:33:dd:d0:7a:cd:0e:78:8d:9b:66:d8:74:a3.Please contact your
system administrator.Add correct host key in
/home/openstack/.ssh/known_hosts to get rid of this message.Offending
RSA key in /home/openstack/.ssh/known_hosts:1
  remove with: ssh-keygen -f "/home/openstack/.ssh/known_hosts" -R 10.3.24.56
RSA host key for 10.3.24.56 has changed and you have requested strict
checking.Host key verification failed.

P.S: I know if I run ssh-keygen -f "/home/openstack/.ssh/known_hosts" -R
10.3.24.56 problem can be solved but than I have to provide password to log
in to VM, but my goal is to use keypairs NOT password.
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