Adjust how default_value is calculated for multi-select drop downs

[eclarizio]

Somewhat related to this PR, but will clean up some of the specs and attempt to force the default_value for multiselect drop downs to behave consistently.

https://bugzilla.redhat.com/show_bug.cgi?id=1576288

Basically, with this change, multiselects driven by automate domains should behave slightly differently. Before, you could select a "nil" value, but that doesn't really make too much sense when you're dealing with a multiselect, because you will have to unselect the nil value after you pick another value. So instead of a nil value being selected, nothing will be selected.

Also, with this fix, within automate, the user can set their default values in a few ways:

dialog_field = $evm.object
dialog_field['values'] = [["foo", "Foo"], ["bar", "Bar"], ["baz", "Baz"]]

dialog_field['default_value'] = "foo" # This method for selecting a single item only
dialog_field['default_value'] = ["foo", "bar"] # The preferred way to select two items
dialog_field['default_value'] = '["foo", "bar"]' # Another way to select two items (though admittedly a bit strange)

dialog_field['default_value'] = "foo, bar" # This DOES NOT work, and the field will end up with "Nothing selected"

@himdel I know this was kinda messy before, hopefully this cleans it up a bit. Think you can review quickly so this can get in for the build?

@d-m-u Can you review please?

@miq-bot add_label gaprindashvili/yes, bug, blocker

@miq-bot assign @gmcculloug

[miq-bot]

Checked commit eclarizio@b73e8f6 with ruby 2.3.3, rubocop 0.52.1, haml-lint 0.20.0, and yamllint 1.10.0
4 files checked, 0 offenses detected
Everything looks fine. 🏆

[d-m-u]

BEAUTIFUL

[JPrause]

@gmcculloug there's a build tomorrow, so if this can be merged, that'd be awesome.

[gmcculloug]

👍

[simaishi]

Gaprindashvili backport details:

$ git log -1
commit f7147ee4f2ab68a93bde0dd5341ed18fa0b78eac
Author: Greg McCullough <[email protected]>
Date:   Mon May 21 14:07:53 2018 -0400

    Merge pull request #17449 from eclarizio/BZ1576288
    
    Adjust how default_value is calculated for multi-select drop downs
    (cherry picked from commit ad699eef42aa47d55bda205f583265a81f3691d0)
    
    Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1581307