Be more descriptive when validating hostname
Currently, we validate hostname with a regular expression. If the regular expression
doesn't match, the host name is considred as invalid, printing an info that
name is invalid. In some environments, it's not that uncommon to include
the '_' in the host name, and it's hard to user to figure out what exactly is wrong with
The new behaviour should be to explicitly mention that the host can't include
`_`, before using the regular expression.