all: make tectonic license and pull secret optional

[philips]

The tectonic license and pull secret are required. If they aren't included we should simply not attempt to install the tectonic manifests. This will enable a few things:

• bootkube can use this repo for cluster spin-up and testing
• kubeadm could use this repo for testing across different environments

[philips]

cc @luxas would you be interested in using this repo for kubeadm self-hosted testing? It would be a good way to start testing that across different repos. Essentially it would be creation of a module like https://github.com/coreos/tectonic-installer/tree/master/modules/bootkube

[klausenbusk]

So with this issue resolved, the installer can be used for setting up a non-tectonic cluster like coreos-kubernetes does?

[s-urbaniak]

@klausenbusk yes, setting tectonic_vanilla_k8s to true will deploy a vanilla k8s cluster only and works for AWS and Azure already today.

It is still a bit bumpy as you need to point to empty files for tectonic_license_path and tectonic_pull_secret_path as tectonic assets are being provisioned (but not deployed) but the basic functionality is there. I will file a follow-up PR which will not require pointing to empty license files/pull secrets (and probably also omitting rendering the tectonic assets at all).