VMs failing to go up with RBD volume + volume_use_multipath

For a VM that's using an RBD volume when `volume_use_multipath` is set to true, VMs will fail to go up.

https://github.com/openstack/os-brick/blob/28ffcdbfa138859859beca2f80627c076269be56/os_brick/initiator/linuxscsi.py#L212-L233

It seems like we always call os_brick with enforce_multipath=True, so if that is set to enabled, it ends up failing for all newly provisioned VMs, even if multipath is not in use/necessary.

Ideally, we should be able to safely ignore it if we're trying to plug a backend that doesn't use/support multipath..