utilities/telegraf-plugins#15: Day/Night Tariff support

The Octopus plugin currently only supports pulling pricing data for STANDARD type tariffs - it doesn't attempt to call /day-unit-rates/ or /night-unit-rates/ at all.

When fetching account info, there's a registers attribute:

"registers":[{
    "identifier":"1",
    "rate":"STANDARD",
    "is_settlement_register":true
}]

The docs say that valid values for rate are

• STANDARD
• ECO7_DAY
• ECO7_NIGHT

So, we probably want to check that and if it's not STANDARD have the tariff fetch draw from the other endpoints instead