Add template.conf that instructs to use in-vm kernel by default

[marmarek]

Switch Whonix templates to use in-vm kernel. This is done by setting
empty value to the "kernel" property (as opposed to leaving it at
default).

Fixes QubesOS/qubes-issues#9570

This depends on few other changes (mostly to allow setting this option in template-whonix repo, instead of in builder-debian directly):

• Make setting template.conf more flexible QubesOS/qubes-builderv2#158
• Support installing kernel in template even for minimal flavors QubesOS/qubes-builder-debian#87
• Remove too strict checking for kernel on startup QubesOS/qubes-core-admin#634 (already merged in R4.3, but wants a backport to R4.2)

And also, it needs adding "install-kernel" option to the template builder config (as "minimal" templates normally do not include the kernel): QubesOS/qubes-release-configs#13

And some more testing, just in case.