A small refactoring of the domain

It fixes some eyesores that have been found in glance domain:
1. Removes duplicate declaration of a varariable 'name'
2. Changes type of several image transitions from 'str' to 'tuple'
3. Sets the correct default values to tags and extra_properties
4. Changes strange constructions like
self.tags = kwargs.pop('tags', None) or [] to
self.tags = kwargs.pop('tags', [])

Change-Id: I9309c7dc70b9910c6c09111f26ee489fa7ab9260
Merge "A small refactoring of the domain"