Make sure the default kickstart path is used when the option is unset

[achilleas-k]

image: allow unset kickstart options where possible

For the AnacondaContainerInstaller, having empty kickstart options when
creating the ImageKind should be possible. The only option that's
required is the Kickstart.Path, which has a default value we can set.

For the AnacondaOSTreeInstaller, at least the OSTree options are
required, so return an error when they're not set, but also set the
Kickstart.Path to the default when it's not already set from the caller.

---

image: drop InteractiveDefaultKickstart in AnacondaContainerInstaller

This isn't used and can cause unnecessary errors when 'img.Kickstart' is
not set.

---

test: instantiate and serialize anaconda manifests without ks options

Test that anaconda manifests can be generated without error when
kickstart options are not set (or when only the required ones are set).
This ensures that no errors or panics occur when the kickstart options
are nil or when the kickstart path is empty.

---

Fixes #763

[achilleas-k]

We should have a lot more tests like these and also reconsider what we set in each constructor. I always liked the idea of having, for example, the NewAnacondaContainerInstaller() take as args all the properties it requires and exposing all other properties publicly so they can be set optionally from the caller. An alternative idea, from @mvo5, was to have the optional properties set by an options struct that's always the last argument and can be nil.

Either way, we sort of forgot about this in some places and I see cases where a required property isn't part of the constructor argument list, making the pipeline generator hard to work with; looking at the struct, you'd have to guess which properties are required and which aren't.

A lot of this will probably go away with otk, so it might not be worth investing into cleaning it all up now.

[mvo5]

Thank you! This is nice, some small nitpicks/suggestions inline for your consideration

[achilleas-k]

Need to get #779 in first, then will rebase this.