This patch implement support for node custom attributes
Filter nodes by specific labels:
fuel2 node list --labels "label_key_1=label_val_1" [..]
List all labels in system or only for specific nodes:
fuel2 node label list --nodes 1 2
Set label(s) for all or only specific nodes:
fuel2 node label set "label_key_1=label_val_1" [..] --nodes 1 2
Delete label(s) by key for all or only specific nodes:
fuel2 node label delete "label_key_1" [..] --nodes 1 2
Implements: blueprint node-custom-attributes
Change-Id: Iaa00219a8e4a9c5fa62df2ce4d59057ef17e89a0