Allow to change the network speed unit

[mathoudebine]

As discussed in #714

Is your feature request related to a problem? If so, please describe the problem.
The network speed units are set automatically to the best available based on the current speed (Mb/s, Gb/s...)

Describe the feature / solution to your problem you'd like
I want to set the network speed unit manually to always be the same unit (like Mb/s)