[legacy] add kubernetes.yml -> kubelet.yaml import logic

[xvello]

What does this PR do?

Extend pkg/legacy and agent import command to import settings from the agent5's kubernetes.yaml settings, enabling the kubelet check and importing the rest in the main datadog.yaml configuration.

Several options are deprecated, added deprecation warning during the import + updated the docs:

The following fields are deprecated and not converted:

• api_server_url, apiserver_client_crt, apiserver_client_key, apiserver_ca_cert: please use kubernetes_kubeconfig_path instead
• use_histogram: please contact support to determine the best alternative for you
• namespaces, namespace_name_regexp: Agent6 now collects metrics from all available namespaces
• label_to_tag_prefix: please specify mapping per label via kubernetes_pod_labels_as_tags
• port: Agent6 default mode is to collect kubelet metrics via the new prometheus endpoints. On clusters older than 1.7.6, manually set the cadvisor_port setting to enable cadvisor collection

Successfully imported the contents of /tmp/agent_test_legacy303237348/kubernetes.yaml into datadog.yaml

Logic imports cadvisor-specific enabled_gauges and enabled_rates options, see DataDog/integrations-core#1339

But as the default configuration ships port: 4194, we cannot detect if the user wants to keep cadvisor mode, or move to the new prometheus logic. The code assumes they want and print a warning giving instructions on how to enabled it back.

[xvello]

Will need to add cadvisor params DataDog/integrations-core#1339, making WIP

[mfpierre]

LGTM ☸️ 📄