fabrics: autoconnect: add service unit for connecting NBFT subsystems

fabrics: autoconnect: add service unit for connecting NBFT subsystems

Create a separate unit file for connecting to NBFT-defined subsystems.
This unit is intended to be called in "post-up" scripts from network
management software if an interface defined in the HFI section of the
NBFT is brought up (L3-configured).

In simple scenarios with just one HFI, this won't be necessary because the
interface must be brought up in the initramfs already. But in multipath
scenarios, the initramfs may choose not to wait for every HFI to come up, and
thus it may be necessary to bring up the secondary connection(s) later on.

Signed-off-by: Martin Wilck <mwilck@suse.com>
[dwagner: use unit options instead of ExecStartPre
          update nvme command line]
Signed-off-by: Daniel Wagner <dwagner@suse.de>