Add a default theme

User Story

Feature: Provide a default theme.
  In order to develop a usable interface,
  as a developer,
  I want to be able to use a flexible default theme.

  Scenario: Use a popular base theme.
   Given I run "drush @aegir pmi eldir"
     Then I should get
     """
     Type        :  theme
     Title       :  Eldir
     Status      :  enabled
     Base        :  Bootstrap
     """

...

Completion checklist

• Tests are implemented and passing
• Code coverage remains 100%
• Linting passes
• Code review by core team member