Use Distinst.validate_hostname to check generated hostname

[davidmhewitt]

Problem

Issues like #599 look like we're managing to generate an invalid hostname on certain hardware.

Proposal

Distinst has a built in method to check a hostname is valid, so we should use that and fall back to just elementary-os or something if it's not valid.

[meisenzahl]

I think that we should also use this in elementary/initial-setup#113

[davidmhewitt]

I don't think initial-setup should depend on distinst just for that. But it's obviously a nice easy thing to use in the installer. It would be pretty simple to re-implement the specification for hostname checking. It's essentially:

• Can only contain alphanumeric characters, periods (.) and hyphens (-)
• Must not start or end with a period or hyphen