Updates to PluginBag, DefaultSinglePluginBag, and DefaultPluginBag

The PluginBag class has two additional abstract methods:
public function getConfiguration();
setConfiguration($configuration);
Both of the default plugin bags provide implementations.
These methods need to set and get the whole set of configuration for the plugin bag as a whole.

In order to add these methods, the DefaultPluginBag::setConfiguration() method is now DefaultPluginBag::setInstanceConfiguration(), since it is per instance.

Finally, the DefaultSinglePluginBag is used when there will only ever be one plugin. However, the constructor takes an array of plugin IDs. It now takes a single plugin ID as a string:

// Before
$plugin_bag = new DefaultSinglePluginBag($manager, array($instance_id), $configuration);
// After
$plugin_bag = new DefaultSinglePluginBag($manager, $instance_id, $configuration);