Description of problem:
Based on  https://docs.jboss.org/author/display/RHQ/Canned+DynaGroup+expressions design document, create a way so plugin can provide predefined dyna-group expressions. 


Version-Release number of selected component (if applicable):
RHQ 4.10

Thinks taken into account (but may change after further discussion)

1. additional plugin descriptor will be 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
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. Now, there is hardcoded list of expressions - this will be replaced by expressions comming from plugin.
5. When user sees/edits dynaGroup which was auto-created because of 'createByDefault=true', he's notified with consequences (3.d,e)