add bypass key cheching ssh-config

Hello,

I have many servers and I can't connect to each of them to validate the first connection. Without it, the restic fails.

I propose two/tree solutions that I can push in PR.

1: add a variable to disable StrictHostKeyChecking and set it to no
2: set the default value StrictHostKeyChecking to the value accept-new, which allows to validate the first key but in case of conflict the connection is rejected, it's a good compromise.

3: Both are also possible

default value:
restic_ssh_StrictHostKeyChecking=True

If
restic_ssh_StrictHostKeyChecking=True

then

Default behavior(no template change)

else if restic_ssh_StrictHostKeyChecking=False

StrictHostKeyChecking no

else if restic_ssh_StrictHostKeyChecking=accept-new

StrictHostKeyChecking accept-new

What do you think?

M.