Improve `OOB` plugin's configuration

[damyan]

The OOB plugin does the matching based on a subnet label configuration. Currently the label is implemented as string.

Better: implement as a key-value map, adding support for multiple labels.

[KapilYadava]

proposal format:

oob-config.yaml

namespace: oob-ns
subnetLabels:
  - subnetName: subnet1
    subnetValue: dhcp1
  - subnetName: subnet2
    subnetValue: dhcp2
  - subnetName: subnet3
    subnetValue: dhcp3

[damyan]

proposal format:

oob-config.yaml

namespace: oob-ns
subnetLabels:

• subnetName: subnet1
  subnetValue: dhcp1
• subnetName: subnet2
  subnetValue: dhcp2
• subnetName: subnet3
  subnetValue: dhcp3

Like it! Can we just switch the naming from subnetName/subnetValue to key/value? Those are labels, they can contain anything. That's different to IPAM plugin where we can filter on a set of subnet names.