Bug 1076126 - RFE: Canned dynaGroup expressions provided by plugin

Initial implementation based on https://docs.jboss.org/author/display/RHQ/Canned+DynaGroup+expressions

1. additional yaml plugin descriptor was introduced - optional for plugin. This descriptor is not touched on agent side at all. 
2. canned expressions are not stored to RHQ database. They're available in server's memory as part of plugin metadata
3. as there is an option for 'createByDefault=true', we're auto-creating dyna-groups based on such expressions.
3.a - dynaGroups are updated as plugin updates expression
3.b - dynaGroups are removed in case plugin no longer provides expression for such dynaGroup (with createByDefault=true)
3.c - dynaGroups are removed in case plugin is deleted
3.d - if user edit's such dynaGroup's properties - it will be overwriten on plugin update
3.e - if user deletes such dynaGroup, it will appear again on plugin update
4. All expressions from all loaded plugin are presented in dropDown on UI page, when creating new dynaGroup. Hardcoded list of expressions was replaced by expressions comming from plugins.

I'd appriciate anyone's feedback especially on chosen file structure/format, dynaGroup update/delete behavior and defaults used.