Add ability to run qbec without kubeconfig and override qbec.yaml

Start by tracing qbec's command-line configuration and Kubernetes credential-loading paths, then compare how qbec currently reads ~/.kube/config and qbec.yaml. Define the supported flag or environment-variable inputs and their precedence before implementing the override behavior. Done means qbec can connect using supplied cluster credentials without those files, with tests covering the new inputs.

Right now, qbec relies on ~/.kube/config and qbec.yaml files when it tries to find cluster credentials.

I think it would be rather nice if qbec will have the ability to provide all the parameters to connect to the cluster via command line.

In my specific case, I want the ability to run qbec from Terraform.
For instance, Terraform providers have configuration like this:

resource "aws_eks_cluster" "cluster" {
  name = "mycluster"
}

data "aws_eks_cluster_auth" "cluster" {
  name = aws_eks_cluster.cluster.id
}

provider "kubernetes" {
  host                   = aws_eks_cluster.main[0].endpoint
  cluster_ca_certificate = base64decode(aws_eks_cluster.main[0].certificate_authority.0.data)
  token                  = data.aws_eks_cluster_auth.cluster.token
}

And I imagine I want to run qbec from terraform with options like this:

resource "aws_eks_cluster" "cluster" {
  name = "mycluster"
}

data "aws_eks_cluster_auth" "cluster" {
  name = aws_eks_cluster.cluster.id
}

resource "null_resource" "cluster" {
  provisioner "local-exec" {
    command = <<COMMAND
qbec \
--k8s:token=${data.aws_eks_cluster_auth.cluster.token} \
--k8s:cluster-server=${aws_eks_cluster.cluster.endpoint} \
--k8s:cluster-ca-certificate=${aws_eks_cluster.cluster.certificate_authority.0.data} \
apply
COMMAND
  }
}

And/or with environment variables like this:

resource "aws_eks_cluster" "cluster" {
  name = "mycluster"
}

data "aws_eks_cluster_auth" "cluster" {
  name = aws_eks_cluster.cluster.id
}

resource "null_resource" "cluster" {
  provisioner "local-exec" {
    command = "qbec apply"
    environment = {
      QBEC_K8S_TOKEN = data.aws_eks_cluster_auth.cluster.token
      QBEC_K8S_CLUSTER_SERVER = aws_eks_cluster.cluster.endpoint
      QBEC_K8S_CLUSTER_CA_CERTIFICATE = aws_eks_cluster.cluster.certificate_authority.0.data
    }
  }
}

Options names are discussable ofc.