Add default Plugin.setup() method

Adding a default setup() method to the base Plugin class allows
useful plugins to be written declaratively by setting the 'files'
and 'packages' members of the subclass, e.g.:

    class FooPlugin(Plugin):
	plugin_name = 'foo'
        packages = ('foo',)
        files = ('/etc/foo.conf',)

Creates a plugin class named 'foo' that will run whenever the
'foo' package is installed or the '/etc/foo.conf' file exists.

The method calls self.add_copy_specs() with the plugin's files
list.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>