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/net_configuration.json

@@ -0,0 +1,10 @@
+{
+ "ipv4_address": "192.0.2.150/24",
+ "ipv4_configuration": "DHCP",
+ "mac_address": "0c:c4:7a:00:00:01",
+ "ipv4_gateway": "192.0.2.1",
+ "ipv4_backup_gateway": null,
+ "ipv4_gateway_mac": "02:00:00:00:00:01",
+ "ipv4_backup_gateway_mac": null,
+ "vlan_id": "off"
+}