Fix netdata system_load and add disk_free.

[michaelarnauts]

Description:

system_load was incorrectly returning the number of processes, I guess due to a copy paste error from processes_running.
I've also added disk_free that fetches the amount of free disk space on the /-mountpoint.

Checklist:

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.github.io

If the code communicates with devices, web services, or third-party tools:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

If the code does not interact with devices:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• Tests have been added to verify that the new code works.

[fabaff]

Looks good to me 🐦