linux: Ignore filesystems in disks usage with zero size

This patch introduces a new option in the linux configuration files
to ignore mounted filesystems with zero size.
We're currently black listing file system types and with every
new distribution we support new filesystem types have to be added
which usually have no size. (total/used size equals 0)

This patch introduces the ignore_zero_size_fs option to allow users
to revert back to the old behaviour by setting the value to `false`.

Change-Id: Iacaf57966aec2d973be39776b5f2457b27dac689
Bug-Url: https://bugzilla.redhat.com/1130993
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>