Use upstream digitalocean provider

ATM we use the provider supporting auto-scaling. I took the following repo/branch: https://github.com/snormore/terraform-provider-digitalocean/tree/doks-autoscale

This is the branch taken from the PR: https://github.com/terraform-providers/terraform-provider-digitalocean/pull/307

When merged and released, we should be able to use the default provider.

Todo:

• Auto-scaling PR has been merged
• A new release (propably v1.9.2 or 1.10.0) has been made containing the autoscaling feature
• Remove /terraform-plugins from repo
• Add correct plugin version in main.tf
• Change .gitlab-ci to not use the -plugins-dir flag when running terraform init