ansible-scylla-node: auto-detect NVMe: only use non-mounted disks.

[vreniers]

NVMe disks should only be selected that are not in use, which should be the case when they are not mounted.

Fixes #407

[tarzanek]

  lsblk -oNAME,PATH,PTTYPE,MOUNTPOINT -nd | grep nvme | awk '{if ($3=="" && $4=="") print $2}'

misbehaves on md0 with multiple nvmes

[vladzcloudius]

  lsblk -oNAME,PATH,PTTYPE,MOUNTPOINT -nd | grep nvme | awk '{if ($3=="" && $4=="") print $2}'

misbehaves on md0 with multiple nvmes

What do you mean exactly, @tarzanek ?
Please, provide the corresponding output so that we can all see what's the problem.