Use fedora-30 for sys-* VMs

[conorsch]

It's that time again! Fedora-29, currently, used for the sys-* VMs (viz. sys-firewall, sys-net, sys-usb) is approaching EOL. As of this writing, Fedora 29 is listed on neither the EOL Fedora info page, nor the supported versions page:

• https://fedoraproject.org/wiki/End_of_life
• https://fedoraproject.org/wiki/Releases

Still, F29's days are numbered, so let's make the jump. In the past, we've recommended that developers make the change manually. We still have #262 looming, however, so I'll take a closer look at what are options are to make these tricky changes in the configuration logic.

Related issues:

• Use Fedora 28 templates #114
• Fedora-26 still used for sys- VMs #189
• Updates for Qubes 4.0.1 #245