Allow required flag to be set/unset for all dialog field types

[eclarizio]

As an add-on to #2185 discovered in the 5.8 clone: https://bugzilla.redhat.com/show_bug.cgi?id=1496946

#2185 fixed the ability to set the required property for dynamic drop down fields, but for some reason the required property was never included in the dialog_get_form_vars_field method for drop downs (as well as checkboxes).

This PR refactors the line for checking the required parameter that gets passed in when you click the checkbox in the UI, and ensures that it saves.

Note that again, the new dialog editor does not have this problem.

Original related BZ: https://bugzilla.redhat.com/show_bug.cgi?id=1492150

/cc @gmcculloug
@miq-bot add_label euwe/yes, fine/yes, bug
@miq-bot assign @h-kataria

[miq-bot]

Checked commit eclarizio@cc4048e with ruby 2.3.3, rubocop 0.47.1, and haml-lint 0.20.0
1 file checked, 0 offenses detected
Everything looks fine. ⭐

[eclarizio]

@miq-bot add_label blocker

[h-kataria]

looks good

[simaishi]

Fine backport details:

$ git log -1
commit e33276cfa07898d792a5257e9d65d32cfd052bff
Author: Harpreet Kataria <[email protected]>
Date:   Mon Oct 9 12:18:33 2017 -0400

    Merge pull request #2313 from eclarizio/allow_required_flag_for_all_dialog_field_types
    
    Allow required flag to be set/unset for all dialog field types
    (cherry picked from commit f40bb3ba78cd2d5e048da06398b46123d37ced2b)
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1496946

[simaishi]

Euwe backport details:

$ git log -1
commit 7a95d49b47b434cbd3d6e7d2b223ebdc53fe33e0
Author: Harpreet Kataria <[email protected]>
Date:   Mon Oct 9 12:18:33 2017 -0400

    Merge pull request #2313 from eclarizio/allow_required_flag_for_all_dialog_field_types
    
    Allow required flag to be set/unset for all dialog field types
    (cherry picked from commit f40bb3ba78cd2d5e048da06398b46123d37ced2b)
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1495971