Prologue: customize background of buttonViewController

[ctarda]

Accept a parameter to change the background of the view that contains the action buttons in LoginPrologueViewController.

This will help with the implementation of woocommerce/woocommerce-ios#3144

Expected changes:

Add an optional parameter to WordPressAuthenticatorUnifiedStyle to signal the background color, defaulting to clear.
Update LoginPrologueViewController to assign the background color passed as parameter