In _populate_custom_choices(), all methods called at the method have
True as a second argument. It means that the default values sepcified
in a stack are not shown as default when launch stack form.
The fileds always show 'Select Flavor/Image/Network'.
Users doesn't expect to choose default value by hand.
This change shows the default values defined at a template.
Also, image_field_data() doesn't use parameter, include_empty_option.
This change modfies the method to change the behavior based on the
parameter.
Closes-Bug: #1619465
Change-Id: I4b6bef322a668a805abcd4bc472d30f6d1df3a0a
(cherry picked from commit 792f8cbe4a)
c8456777df