Add http-01 support to the Let's Encrypt mechanism.

[knightsc]

Since the autocert.Manager is always created regardless of whether
Let's Encrypt is being used or not I opted to always wrap the HTTP
redirect handler with the autocert.Manager HTTPHandler. This means
that port 80 will always be available to handle a Let's Encrypt challenge
as well as do the redirect. This should get Let's Encrypt support
functioning again since tls-sni-02 and tls-sni-01 were deprecated.

Fixes micromdm/micromdm#408

Testing this was a little bit of a pain but here's the process I followed.

• Spun up a new EC2 instance (or really anything with a public IP would work)
• Put a build of micromdm with these changes on the server
• Created a public DNS entry to point to the public EC2 server
• Ran micromdm as follows sudo ./build/darwin/micromdm serve --server-url https://mydomain
• Tried accessing https://mydomain in a web browser

The result was no errors like mentioned in micromdm/micromdm#408 and I could view the certificate from the http response and it was indeed one issued by lets encrypt.

[knightsc]

I think I'll need to refactor this a little since currently this code will only run if redirectHTTPS is true and we might be using Let's Encrypt but not doing redirects in that case these changes wouldn't work.

[knightsc]

Added a fixup in 178158e. If redirectHTTPS is true or we've set the GetCertificate config which indicates we're using Let's Encrypt then we will start the http listener. Using the redirect methods in both cases should be fine because the autocert.HTTPHandler will do a redirect anyway if nil is passed in which is the other option we could do. I think this should correctly work now in all cases