fix ssh_authorized_keys

[kral2]

This change simplify how ssh_authorized_keys is handled and support more scenarios.

The module input variable now expect a string. It gives more flexibility to the module user to construct the string as needed: heredoc, file function ...

Usage is explained in a new documentation file: docs/instance_ssh_keys.adoc

All the use cases can be handled by only one variable: ssh_public_keys (Changing to plural form regarding fc662062bb7890e6782096005ee109e9696d04d0).

resource "oci_core_compute" "my_instance" {
...
  metadata = {
    ssh_authorized_keys = var.ssh_public_keys != null ? var.ssh_public_keys : file(var.ssh_authorized_keys)
    user_data           = var.user_data
  }
...
}

The conditional is here only for backward compatibility with var.ssh_authorized_keys. As soon as we move to the next major release, we can drop the conditional all together and adopt this simpler form:

resource "oci_core_compute" "my_instance" {
...
metadata = {
    ssh_authorized_keys = var.ssh_public_keys
    user_data           = var.user_data
  }
...
}

The module user will assign value to this argument like this:

module "my_instance" {
...
  ssh_public_keys = var.my_public_ssh_key
...
}

To provide multiple keys at once, just use Heredoc strings:

module "my_instance" {
...
  ssh_public_keys = <<EOF
<public ssh key 1>
<public ssh key 2>
<public ssh key n>
EOF
...
}

If the module user prefer to provide keys from a file, that's also possible:

module "my_instance" {
...
  ssh_public_keys = file("/path/to/file")
...
}

Fix #70