Add DNS provider for Combell (reseller only)

[ldez]

Closes #1573

@jwaes can you run the command with your domain, email, credentials, etc. (and put the output of this command to a comment)?

make build
rm -rf .lego

COMBELL_API_KEY="xxx" \
COMBELL_API_SECRET="yyy" \
./dist/lego run --dns combell -d '*.example.com' -d example.com -s letsencrypt-staging

Note that the wildcard domain is important.

How to test this PR?

1. You need Go
2. Check out the PR:

   git clone https://github.com/ldez/lego.git
   cd lego
   git checkout ldez:feat/combell

3. Compile lego:
   • if you have make: make build
   • if you don't have make: go build -o dist/lego ./cmd/lego
4. Run the following command with your information (email, domain, credentials):

   COMBELL_API_KEY="xxx" \
   COMBELL_API_SECRET="yyy" \
   ./dist/lego run --dns combell -d '*.example.com' -d example.com -s letsencrypt-staging

   The wildcard domain is important
5. Before each run of the command, you should clean your local environment:

   rm -rf .lego

[jwaes]

blush ... now it turns out, that the combell API is only for resellers, not for 'ordinary' customers.
I guess i ll have to switch to a decent provider. Sorry for the work you put in.

[ldez]

Hello @jwaes, it seems like Combell has another API, related to OpenStack, if it's true maybe you can use Designate DNSaaS for Openstack.