Network interface speed

Created by: horazont

See SovereignCloudStack/Docs#98 for my definition of Prio 1-4.

• Prio 3: As a Cloud Operator, I want to know when a network interface drops below the specified maximum speed on that link, as that indicates bad cabeling, a bad driver, or another (hardware) issue and may cause customer impact (reduced performance).
• Prio 2: As a Cloud Operator, I want to know when the network interfaces of a compute node are saturated, as that causes degration of performance.