Retrieve system parameters from servers

Implement a method that retrieves certain system parameters from the server and stores them on the server object (as a dictionary).

total/used/available memory
total/used/available CPU
system load
total/used/available disk space

Also, add a wrapper method on BigBlueButtonGroup that aggregates the stats from the member servers, and a bbb-cli subcommand that prints a table of the stats for all member servers.

Depends on #10 (closed).

Edited May 17, 2020 by Nik | Klampfradler

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