Prepare hub before terraforming other clusters

[charlieegan3]

We had an issue (#377) where weneeded to plan the hub just to get the local files in place before running operations against other clusters.

This 'Prepares' the hub cluster before verifying it which puts the required files in place.

If the hub has not been applied, then trying to terraform another cluster in the env will still fail.

I've also added a comment explaining this function a little more as it's caused me some confusion in the past.

What this PR does / why we need it:
Fixes #377

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #377

Special notes for your reviewer:

Release note:

Prepare Hub clusters for output before terraforming other clusters in a multicluster environment. Fixes need to plan against hubs before applying.

[charlieegan3]

Provided for interest. This is the error you get if the hub has not been applied:

DEBU[0014] Terraform has been successfully initialized!  app=tarmak module=terraform std=out
ERRO[0017] The state file either has no outputs defined, or all the defined
outputs are empty. Please define an output in your configuration
with the `output` keyword and run `terraform refresh` for it to
become available. If you are using interpolation, please verify
the interpolated value is not empty. You can use the 
`terraform console` command to assist.
  app=tarmak module=terraform
FATA[0017] Tarmak exited with an error: failed to validate tarmak cluster: 1 error occurred:

* failed to get hub cluster output values, hub cluster must be applied once first: error getting terraform output: exit status 1  error="failed to validate tarmak cluster: 1 error occurred:\n\n* failed to get hub cluster output values, hub cluster must be applied once first: error getting terraform output: exit status 1"

It's not ideal but at the end it does say hub cluster must be applied once first

[charlieegan3]

/assign @simonswine

[simonswine]

/approve
/lgtm

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