In case some mounts are present for systems out of the box, it
might be still important to apply overrides for it, like custom options.
For that we add new `mount_overrides_only` key to `systemd_mounts`
mapping, alike to how it is already done for networkd and serviced.
Change-Id: Ib421508cd76147cb314a579a2b4e918604cf3652
Signed-off-by: Dmitriy Rabotyagov <dmitriy.rabotyagov@cleura.com>
feb10e7629