Add hostname config for GitLab Pages

What does this Merge Request do?

Currently, GDK depends on xip.io and it means Pages won't work while offline. GitLab Pages allows specifying a custom domain by using -pages-domain config flag.

This MR intends to add support to specifying a gitlab_pages.hostname via the gdk.yml file. It also updates the documentation with simple instructions on how to make a custom domain work locally by editing /etc/hosts manually. It also points to dnsmasq for wildcard domains.

Closes #967 (closed)

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.