Skip certificate management if none given

[Tangresh]

What does this PR fix?

If a certificate_authorities is configured by setting ssl_cert_path but no certificate file exists at salt://filebeat/files/ca.pem, the file.managed state in filebeat.config fails.

Solution

This PR brings the possibility to disable the management of a logstash certificate authority file by setting ssl_cert to None:

filebeat:
  logstash:
    tls:
      ssl_cert:

This is not the nicest solution to the problem, but it does not break existing configurations.

Additional Contents

This PR also adds the ability to disable using custom certificate_authorities, which is also attempted with #11, which seems to be dormant. To disable custom certificate_authorities, set ssl_cert_path to None:

filebeat:
  logstash:
    tls:
      ssl_cert_path:

[gravyboat]

Good stuff, thanks.