Set default columns in ext-list

Previously ext-list does not inherits ListCommand base class for listing
and cannot specify columns to display. This commit changes ext-list to
use ListCommand and adds a unit test for extensions.

This also changes the default value of _formatters in ListCommand to {}.
By this subclasses no longer override _formatters unless required.

Fixes bug 1161866

Change-Id: Ifd9ab54d4e84a2b7a1d7eecd67f6270ec643db1d
Merge "Set default columns in ext-list"