ydns: embed hostname directly in script

This will allow to call the script from different points.
ydns: also update on DHCP renew
ydns: don't update IPv6

When an host has both IPv4 and IPv6, we need to specify the record
ID. This is cumbersome. Moreover, we cannot disable IPv6 if not
available. Just update IPv4 for now.
ddns: switch to Route53

Not free, but we can use IPv6 as I expect to use it.
ddns: fix connection to Route53 to use credentials
ddns: avoid updating DNS on all hosts at once