setup: fix alpine ipv6-slaac configuration

setup: fix alpine ipv6-slaac configuration

busybox ifupdown implementation differs from debian's - configuration type
auto is not supported. If SLAAC is selected for the ipv6 configuration of an
interface, the complete networking is not started, because of that error.

This workaround sets the interface type to 'manual' in case SLAAC is selected
(as is already done for dhcpv6 (for different reasons)). That way all other
configuration stanzas are setup correctly, and if a ipv4 configuration is
present for the same interface the SLAAC-part usually works out of the box
anyways (unless 'accept_ra' is set to 0 for the interface in the kernel).

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>