Remove/Modify auto-generation of config.yaml in empty directory

[ishantanu]

Describe the bug
When tk8 is run in a directory which does not have config.yaml, it tries to auto generate it by saying:

No default config was provided. Generating one for you.

The auto-generated config.yaml is only usable for aws provisioner and not for others.

To Reproduce
Steps to reproduce the behavior:

1. Download tk8 binary in empty folder.
2. Setup cattle-aws cluster without config.yaml: tk8 cluster install cattle-aws

Expected behavior
We have two options in this case:

1. Instead of auto-generating config.yaml, we should tell the user to create the config.yaml before running the command since we cannot generate multiple configs for multiple provisioners.

2. Implement the code for mapping between different config.yaml's for different provisioners.