Cancel signing request with IP instead of Domain

[gjongenelen]

Sometimes I see errors in my logs with a failed domain signing request
"detail": "Error creating new authz :: Issuance for IP addresses not supported"

Could you make a check on the beginning of the sequence to check wether it's a domain or IP, and cancel the sequence when it is a IP-address? Just to beautify the logs.

[luto]

Thanks for you suggestion. We've already got an issue covering it: #26

@GUI or @gjongenelen please close this :)