Refactor Terraform executor

[johananl]

This PR paves the way towards #716. By making terraform.Executor accept an arbitrary series of "steps" we can now declaratively define the infrastructure pieces a platform is composed of.

Example:

steps := []terraform.ExecutionStep{
    {
        Description: "create controllers",
        Args: []string{
            "apply",
            "-auto-approve",
            fmt.Sprintf("-target=module.packet-%s.packet_device.controllers", c.ClusterName),
        },
    },
    {
        Description: "create DNS records",
        Args:        []string{"apply", "-auto-approve", "-target=module.dns"},
    },
    {
        Description: "refresh Terraform state",
        Args:        []string{"refresh"},
    },
    {
        Description:      "complete infrastructure creation",
        Args:             []string{"apply", "-auto-approve"},
        // Verify DNS entries were manually created by the user before proceeding.
        PreExecutionHook: dns.ManualConfigPrompt(&c.DNS),
    },
}

I've deliberately avoided refactoring all the various platforms to keep the scope of this PR narrow. I've only refactored platform/packet (which is our most complex platform implementation at the moment) to demonstrate how the new structure can be used. The rest of the platforms will be refactored in subsequent PRs.

[invidian]

Nice PR @johananl 👍

[invidian]

LGTM, though it is possible to do even more fair-grained commits, for example:

• pkg/dns: handle stdin read errors
• pkg/terraform: add OutputBytes function (to unexport executeSync)
• pkg/dns: use terraform.OutputBytes instead of terraform.ExecuteSync
• pkg/terraform: add ExecutionStep concept
• pkg/dns: add ManualConfigPrompt function
• pkg/platform/packet: switch to use terraform.ExecutionStep
• pkg/terraform: unexport low-level functions

I added comments regarding some nits, but they are not very important for the scope of this PR.

[invidian]

LGTM

[surajssd]

LGTM