Control default routes on boot

[sjpb]

Adds a gateway_ip OpenTofu variable which can be used to ensure (only) a default route via the specified IP exists.

Tested that:

• it can add a default route if none is provided by the subnet
• it can remove a default route on a dual-homed host where both subnets provide one
• it makes no changes if the specified default route matches the subnet provided one

[sjpb]

Image build: https://github.com/stackhpc/ansible-slurm-appliance/actions/runs/13809298235

[sjpb]

Image build: https://github.com/stackhpc/ansible-slurm-appliance/actions/runs/13810727551

[sjpb]

Image build: https://github.com/stackhpc/ansible-slurm-appliance/actions/runs/13813989906

[jovial]

All looks sensible to me