Support filesystem customizations in Anaconda-based ISO image types

[achilleas-k]

We should support converting filesystem customizations from the blueprint to Anaconda kickstart partitioning commands that will create the desired mountpoints at install time.

Full support for all the partitioning options supported by kickstart is not necessary (and probably not feasible), but basic partitioning at the extent supported for other image types should be added.

We very often get questions from users about the lack of filesystem customizations when building an Anadonda-based ISO with a payload (the image-installer image type). Sometimes these issues manifest as a request to automate the installation #2255 and other times it's confusion about the customization being ignored #2255 (comment).

A side issue is producing an error in cases where customizations aren't supported (see #3435).