Fix the sub-workflow user input handling

[dgdavid]

Problem

the installer does not go back beyond the "Add On Product" step

• Bugzilla: https://bugzilla.suse.com/show_bug.cgi?id=1156528
• Trello: https://trello.com/c/tOe99fFW/1457-sles15-sp2-p2-1156528-the-installer-does-not-go-back-beyond-the-add-on-product-step

Reason

Changes made in #78 to not abort the installation when an EULA add-on is refused breaks the sub-workflow without propagating the action first when the user is going :back.

Solution

Propagate the user input properly, without forcing to break the workflow. Actually, it is already being interrupted when the action is :back (among others).

Notes

I was also tempted to improve a little bit the code changing the if statement by a case one, which would allow to include the ret = ret2 assignation when the ret2 == :back, reducing the complexity (a little bit).

But I discarded the idea since it's out of the scope of this fix.

Test

Tested manually via dud.

Screencast

Click to show/hide a short screencast

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[coveralls]

Coverage remained the same at 18.805% when pulling 44a9d5b on fix_add_on_subworkflow into 02a1875 on SLE-15-SP1.