Convert manual to Webroot renewal

[schoen]

@ramzdam

Plugins selected: Authenticator webroot, Installer None
Starting new HTTPS connection (1): acme-staging-v02.api.letsencrypt.org
Renewing an existing certificate
Performing the following challenges:
http-01 challenge for xxx.com
Client with the currently selected authenticator does not support any combination of challenges that will satisfy the CA. You may need to use an authenticator plugin that can do challenges over DNS.

@bmw, how can this happen? Shouldn't http-01 always be permitted, and always be supported by webroot?