install release keys in a saner way

install release keys in a saner way

apt actually expects single exported keys in the trusted
directory, not keyrings. recent gpg2 versions (like that in
Debian Stretch) switch to a different default keyring format
which apt does not handle at all, so the old hack will break
soon.

by changing the key format in this repository from armored
exported public key to binary exported public key, which
both apt in Debian Jessie and apt in Debian Stretch
understand, we can just install those two files directly
in the trusted dir.

bonus: the package content does not change based on gpg
version or configuration anymore.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
CC: dietmar@proxmox.com
CC: w.bumiller@proxmox.com