Proposal for BYO deployment

[FlorinPeter]

The motivation behind this PR is to be able to solve our security requirements by using our own Terraform files and plugins for the supported platforms.
The use of self defined Terraform files should always respecting the boundaries given by the installer.
If the byo is set via the install-config.yaml then the installer will not unpack the platform specific files and expects them in the specified installer directory.

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[DanyC97]

@FlorinPeter wouldn't the output generated by the installer be enough to then deploy the cluster on top of your infrastructure created with your self defined Terraform files?

i'm trying to understand why the installer should extend its scope to allow custom terrafform files, what is the benefit ? is that going to be the case for majority of BYO users ?

[FlorinPeter]

@DanyC97 from my point of view it is also a possibility to only consume the generated ignition files but the installer is currently not only generating the ignition files.
There is also some logic that waits for till the bootstrap is done and and destroy it afterwards or add the router certificate.
This means that all this logic needs to be self organized in a BYO scenario.
In the end the effort to write a copy of this installer is much more then to allow the users be able participate from this logic that is already here.
For sure there are some boundries that are given by the installer but for my point of view it is much less effort to allow to overwrite the baked in Terraform files instead of writing a copy of this installer.
This feature will enable users to have customized deployments especially for OpenStack as there are a lot of variations out in the field which requires sometimes different settings for dns or loadbalancing.

[FlorinPeter]

close in favor of #1393