* Beats enrollment subcommand (elastic#7182)

This PR implements intial enrollment to Central Management in Kibana. After running the enrollment command, beats will have a valid access token to use when retrieving configurations.

To test this:

- Use the following branches:
  - Elasticsearch: https://github.com/ycombinator/elasticsearch/tree/x-pack/management/beats
  - Kibana: https://github.com/elastic/kibana/tree/feature/x-pack/management/beats
- Retrieve a valid enrollment token:
```
curl  \
  -u elastic \
  -H 'kbn-xsrf: foobar'  \
  -H 'Content-Type: application/json' \
  -X POST \
  http://localhost:5601/api/beats/enrollment_tokens
```
- Use it:
```
<beat> enroll http://localhost:5601 <enrollment_token>
```
- Check agent is enrolled:
```
curl http://localhost:5601/api/beats/agents | jq
```

This is part of elastic#7028, closes elastic#7032

* Add API client to retrieve configurations from CM (elastic#8155)

* Add central management service (elastic#8263)

* Add config manager initial skeleton

Config manager will poll configs from Kibana and apply them locally. It must be
started with the beat.

In order to check the user is not trying to override configurations
provided by central management, the Config Manager can check the exisitng
configuration and return errors if something is wrong.

* Register output for reloading (elastic#8378)

* Also send beat name when enrolling (elastic#8380)

* Refactor how configs are stored (elastic#8379)

* Refactor configs storage to avoid YAML issues

* Refactor manager loop to avoid repeated code

* Use beat name var when registering confs (elastic#8435)

This should make Auditbeat or any other beat based on Metricbeat have
their own namespace for confs

* Allow user/passwd based enrollment (elastic#8524)

* Allow user/passwd based enrollment

This allows to enroll using the following workflow:

```
$ <beat> enroll http://kibana:5601 --username elastic
Enter password:

Enrolled and ready to retrieve settings from Kibana
```

It also allows to pass the password as an env variable:

```
PASS=...
$ <beat> enroll http://kibana:5601 --username elastic --password env:PASS

Enrolled and ready to retrieve settings from Kibana
```

* Fix some strings after review comments

* Add changelog

(cherry picked from commit 4247bc3)