Support reading perimeter config for "fleet"

[hansr]

We want to default a fleet to 0x60... for all perimeters without a fleet setting, but we need to add a "fleet" setting for perimeters that all devices using that fleet will use for comms.

I'm not sure if the perimeter settings have a different area for storing/providing settings than do k/vs. e.g. unsure if smart contract work is required.

ZTNA UI will have a way to set fleet address for a perimeter.

Ref https://github.com/diodechain/ztna-dev/issues/88

[tuhalf]

I will add a "default" tag inside the contract. If a tag and value is not set for a perimeter, we will return the default value. We will use that for fleet perimeter.