Refactor plugin configuration

Refactor plugin configuration

Fork a PluginConfig class out of PluginObject, the base object now supports
a simple dictionary config, while using PluginConfig provide access to
structured util.config based configuration.

Change UI code that deal with plugins configuration to properly use the new
structured config objects in order to represent data in appropriate format
based on the data type.

Use the new util.config objects to represent plugins configuration.

Signed-off-by: Simo Sorce <simo@redhat.com>
Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>