Fix horizon.tables.Column __init__ method.

__init__ method currently does not work (raises an exception)
when 'transform' arg is callable and 'verbose_name' arg is absent.
Fixed __init__ to use "" as default for 'verbose_name' when
'transform' is callable.

Change-Id: I45c7cd4502e7ea8c809f05ff0ed70b4f766c17fd
Closes-bug: #1230400
Merge "Fix horizon.tables.Column __init__ method."