Add support for Certbot dns_cloudflare plugin.

[nick-dasilva]

Affected Puppet, Ruby, OS and module versions/distributions

• Puppet: all
• Ruby: all
• Distribution: all
• Module version: all

How to reproduce (e.g Puppet code you use)

Set letsencrypt::certonly::plugin parameter to 'dns-cloudflare'.

What are you seeing

Puppet run fails due to 'dns-cloudflare' not being a valid enum value in Letsencrypt::Plugin.

What behaviour did you expect instead

Puppet run succeeds without error, and DNS-01 challenge is issued.

Output log

Any additional information you'd like to impart