Add DNS provider for Combell (reseller only)

[ldez]

• have a homogeneous design with the other providers
• pass the linter
• do go mod tidy
• add tests (units)
• add tests ("live")
• add a provider descriptor
• generate CLI help, documentation, and readme.
• be able to do: (and put the output of this command to a comment)

rm -rf .lego/

COMBELL_API_KEY=xxx COMBELL_API_SECRET=yyy ./lego -m your_email@example.com --dns combell -d *.example.com -d example.com -s https://acme-staging-v02.api.letsencrypt.org/directory run

Note the wildcard domain is important.

@jwaes could you test by running the previous command (you have to use a real domain and email)?

Closes #1573

[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.