support multiple ssh keys

[sudoapt-getclean]

I needed support for multiple keys, the cleanest way to do that seemed to me to just change the existing var.ssh_keys to a list

An alternative implementation that would be backwards compatible would be to introduce a separate variable ssh_keys and merge them in a local. let me know if you prefer that.

[ghost]

All CLA requirements met.

[yupwei68]

Hi @sudoapt-getclean , thanks for opening this PR.

1. I prefer not to break the users by add an extra variable "ssh_keys"
2. Please enabled this feature into fixture/test/main.tf and test if it works when it's assigned and if it works when it's not assigned
3. Please add this feature into the example of README.md

[yupwei68]

Thanks! LGTM!