Discover Flatcar Linux properly for hostname (#69627)

To avoid issues with Flatcar Container Linux being unable to be found,
detect Flatcar distro name especially for hostname, just like CoreOS
Container Linux was supported.

See also https://github.com/ansible/ansible/issues/69516

hacking/tests/gen_distribution_version_testcase.py

@@ -37,6 +37,7 @@ filelist = [
     '/etc/altlinux-release',
     '/etc/os-release',
     '/etc/coreos/update.conf',
+    '/etc/flatcar/update.conf',
     '/usr/lib/os-release',
 ]