dracut: do not install dracut module in non-hostonly by default

[jozzsi]

In non-hostonly (generic) dracut invocation, only add rdma dracut module to the generated initramfs if it is explicitly requested.

[jozzsi]

There is very little testing and documentation (and willingness to improve - see https://github.com/linux-rdma/rdma-core/pull/1649) of the rdma dracut module in non-hostonly mode. Let's at least do not install it by default.

[jgunthorpe]

I'm OK with this idea, if someone wants to support host only options for the special cases that need rdma to be setup from the initrd they can come later and figure out what that means for them. It is a pretty rare config already, I think.