Support arm64 installation of etcd

[AndiDog]

Towards #90

Fix hardcoded amd64 and instead just look up the architecture of the host. Very likely, this also supports further CPU architectures. If Go were installed, we could use go env GOARCH, but it isn't and therefore we have to replace the output of the built-in arch binary (similar/equal to uname -m I think) to match Go's architecture names.

This changes the script from a Terraform template file to a plain file. It instead takes the etcd version as argument, which 1) makes local testing easier and 2) you don't have to escape $ to $$ because of Terraform.

[pstadler]

Thanks for the explanation. Interestingly, arch on MacOS yields arm64. Probably different between *NIX and *BSD.