secrets.yaml is overwritten by --demo-mode

[fabaff]

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Home Assistant release (hass --version):
0.57.0.dev0

Python release (python3 --version):
Python 3.6.3

Description of problem:
It seems that running hass --demo-mode is overwriting the secrets.yaml file while the known_devices.yaml is preserved.

Expected:
Keep the entries in secrets.yaml.