Unify how are allowed options checked in distro implementation in
relation to Ignition customization. Specifically, delete `HasIgnition()`
function and replace its use by `GetIgnition()` call and checking if it is
`nil`. This approach is consistent with how this is checked for other
customizations.
Signed-off-by: Tomáš Hozza <thozza@redhat.com>
0e4dabd265