feat: read-only IPMI driver; report a missing Redfish license (0.2.0)

A Supermicro X10DRL-i refuses Redfish without a license (SUM, DCMS or
OOB), even the service root. IPMI answers without one.

- New entry point `ipmi` (napalm_redfish.ipmi.IpmiDriver) on
  ManagementControllerDriver, over pyghmi: facts and inventory from the
  FRU (serial number, system UUID), the BMC's LAN interface, sensors
  (temperatures, fans, power supplies), the power state, the event log
  and the BMC firmware. Read-only: no power control, no BMC reset.
  Tested against recordings of the X10 with serials, the UUID, MACs and
  IPs replaced.
- RedfishDriver.open() no longer fails on a license refusal: get_facts
  returns what is known without Redfish, get_device_warnings reports
  bmc_license_required with the licenses the BMC names, every other
  getter raises. netOrk then shows the reason instead of an offline BMC.
- IpmiDriver is not exported from the package: NAPALM's own
  get_network_driver("redfish") takes the first driver class of
  napalm_redfish.

tests/fixtures/supermicro-x10drl-i-3.91/ipmi/inventory.json

@@ -0,0 +1,50 @@
+[
+ [
+  "System",
+  {
+   "Board manufacture date": null,
+   "Board manufacturer": "Supermicro",
+   "Board product name": "X10DRL-i",
+   "Board serial number": "ZM000S000001",
+   "Board model": "X10DRL-i",
+   "Board FRU Id": null,
+   "board_extra": [],
+   "Manufacturer": "Supermicro",
+   "Product name": "X10DRL-i",
+   "Model": "X10DRL-i",
+   "Hardware Version": null,
+   "Serial Number": "ZM000S000001",
+   "Asset Number": null,
+   "FRU ID": null,
+   "product_extra": [],
+   "firmware_version": "3.91",
+   "Device ID": 32,
+   "Device Revision": 1,
+   "Manufacturer ID": 10876,
+   "Product ID": 2097,
+   "oem_parser": null,
+   "UUID": "00000000-534D-C40C-0000-000000000001"
+  }
+ ],
+ [
+  "BMC FRU",
+  {
+   "Board manufacture date": null,
+   "Board manufacturer": "Supermicro",
+   "Board product name": "X10DRL-i",
+   "Board serial number": "ZM000S000001",
+   "Board model": "X10DRL-i",
+   "Board FRU Id": null,
+   "board_extra": [],
+   "Manufacturer": "Supermicro",
+   "Product name": "X10DRL-i",
+   "Model": "X10DRL-i",
+   "Hardware Version": null,
+   "Serial Number": "ZM000S000001",
+   "Asset Number": null,
+   "FRU ID": null,
+   "product_extra": [],
+   "oem_parser": null
+  }
+ ]
+]