Support for block device mounted on multiple nodes.

[kmova]

Example: a single device being mounted in 2 nodes. (SAS Backends), or a network-attached device.

In the current behavior, the last node that discovers a block device, will assume the ownership of the block device and sets its own node-id as the owner.

One option to enhance this functionality is to:

• add a check in the discovery probe to ensure that - the node that is currently assigned still exists and device is visible. (via an gRPC API)
• if device is visible - then set a property on the device to indicate it is shared. The shared attribute can be a list of node specs.
• Add the device into the shared spec.

also handle the case where the previously added node owner is taken out of the cluster - one of the remaining nodes can assume the leadership.

As part of exporter - the nodes can push the metrics of the devices that they own.