[RM-18164] Use /etc/os-release if platform.linux_distribution() empty

[RM-18164] Use /etc/os-release if platform.linux_distribution() empty

On systems with only /etc/os-release (rather than distro-specific
release files), platform.linux_distribution() returns ('', '', '').
In this case, fall back to parsing /etc/os-release instead.

It would better to flip the ordering, and use /etc/os-release first,
with platform.linux_distribution() as a fallback.  There's tests for
seven different distros' os-release files already, but we may want
more before flipping this around.

(It would be better still to just use the distro.linux_distribution()
function, especially since platform.linux_distribution() is
deprecated, but I don't know that there's a way of ensuring that
module is present in advance on all the remote hosts.)

Fixes: http://tracker.ceph.com/issues/18164
Signed-off-by: Tim Serong <tserong@suse.com>