Permit Redfish to enable/disable the network interface card

The InterfaceEnabled property for an EthernetInterfaces element was
hardcoded to "true". This change gets the actual state of the NIC. It
also permits the NIC to be enabled or disabled.

Tested:
GET sut_ip/redfish/v1/Managers/bmc/EthernetInterfaces/eth0 (and eth1)
PATCH InterfaceEnabled to false
GET sut_ip/redfish/v1/Managers/bmc/EthernetInterfaces/eth0 (and eth1)
PATCH InterfaceEnabled to true
GET sut_ip/redfish/v1/Managers/bmc/EthernetInterfaces/eth0 (and eth1)
Confirmed the NIC was disabled from the BMC console using "ip link"
Confirmed the Get NIC status reported the correct enabled state.

Performed the tests above on a different network to confirm the second
NIC can be controlled orthogonally.

Passed service validator.

Change-Id: I09b703118fe71765c7b1020688a803c74648c7c4
Signed-off-by: Johnathan Mantey <johnathanx.mantey@intel.com>