Use dashes instead of underscores for iocage hostnames

Creating multiple plugins iocage uses an underscore, including in the hostname.

Underscores are invalid for hostnames.

https://en.wikipedia.org/wiki/Hostname#Restrictions_on_valid_hostnames 

The Internet standards (Requests for Comments) for protocols mandate that component hostname labels may contain only the ASCII letters 'a' through 'z' (in a case-insensitive manner), the digits '0' through '9' and the hyphen-minus character ('-')