StrictHostKeyCheking no in your /etc/ssh/ssh_config file, where it will be a global option used by every user on the server. Or set it in your
~/.ssh/config file, where it will be the default for only the current user. Or you can use it on the command line via
ssh -o StrictHostKeyChecking=no -l $user $host.
Here's an explanation of how this works from
If this flag is set to “yes”, ssh will never automatically add
host keys to the $HOME/.ssh/known_hosts file, and refuses to
connect to hosts whose host key has changed. This provides max-
imum protection against trojan horse attacks, however, can be
annoying when the /etc/ssh/ssh_known_hosts file is poorly main-
tained, or connections to new hosts are frequently made. This
option forces the user to manually add all new hosts. If this
flag is set to “no”, ssh will automatically add new host keys to
the user known hosts files. If this flag is set to “ask”, new
host keys will be added to the user known host files only after
the user has confirmed that is what they really want to do, and
ssh will refuse to connect to hosts whose host key has changed.
The host keys of known hosts will be verified automatically in
all cases. The argument must be “yes”, “no” or “ask”. The
default is “ask”.