Fix hostname when get_distribution_version() returns a string.

The distro version is a string ('jessie/sid') on Debian unstable and testing.
Because load_platform_subclass() evaluates every subclass as it loops,
comparing numbers when get_distribution_version() does not return a number will
break.

This patch fixes that by returning a number (`0`) when we have an invalid
version, instead of returning `None` or a string.