[Feature Request] Add support for setting the prefered Challenge

[ThaDaVos]

Currently it seems the call to Certbot is hardcoded:

docker-easy-haproxy/src/functions/__init__.py

Lines 339 to 354 in 1a68534

	certbot_certonly = ('/usr/bin/certbot certonly {acme_server}'
	' --standalone'
	' --preferred-challenges http'
	' --http-01-port 2080'
	' --agree-tos'
	' --issuance-timeout 90'
	' --no-eff-email'
	' --non-interactive'
	' --max-log-backups=0'
	' {eab_kid} {eab_hmac_key}'
	' {certs} --email {email}'.format(eab_kid=self.eab_kid,
	eab_hmac_key=self.eab_hmac_key,
	certs=' '.join(request_certs),
	email=self.email,
	acme_server=self.acme_server)
	)

Could this be opened up so one can configure the parameters on a container basis with labels? In my case, I want to change the preferred challenge from HTTP to DNS and use acme-dns to resolve it:
https://github.com/joohoi/acme-dns
With hook:
https://github.com/koesie10/acme-dns-certbot-hook

[ThaDaVos]

Created a PR: #57