feat(k3s_env_vars): introduce k3s_install_env_vars

[FalcoSuessgott]

This PR introduces a variable k3s_install_env_vars which allows the user to specify a map of strings that is used to k3s installation variables (https://docs.k3s.io/reference/env-variables).

This allows me to make settings like these:

module "k3s" {
  source = "<URI>"

  k3s_version = "latest"
  
  k3s_install_env_vars = {
    INSTALL_K3S_BIN_DIR = "/usr/bin"
  }
 
 ...
}

Note:

• This would remove the INSTALL_K3S_SELINUX_WARN variable.
• INSTALL_K3S_VERSION can still be set via k3s_version and is checked using variable validation.

[xunleii]

Hi @FalcoSuessgott, thanks for your first contribution to this project.
I just have a suggestion for the validation message and would like to know if the .tfvars is intended or if it is an error.
If it is, I'd rather have the default values directly in the variable (default) and not use a tfvars inside a module.

[FalcoSuessgott]

If it is, I'd rather have the default values directly in the variable (default) and not use a tfvars inside a module.

Good catch. That was indeed just for developing purposes. I removed it now :)

[xunleii]

LGTM, thanks 😄

I'll try to make a release today with this change (and other ones that a forgot to release)