Delegate DSL method values to their superclass

When inheriting from +Resource::LWRPBase+, the +default_action+ and +actions+
attributes are set as class instance variables. In Ruby's object model, class
instance variables are **not** shared with children. So subclassing a resource
that extended from +Resource::LWRPBase+ would result in some unexplainable
side-effects:

  class Chef::Resource::Parent < Chef::Resource::LWRPBase
    actions :execute
    default_action :execute
  end

  class Child::Resource::Child < Chef::Resource::Parent
    # In here, +actions+ and +default_action+ are **not** inherited so they
    # are +nil+!
  end

This commit tells resources that inherit from LWRPBase to check their
superclasses for values if they do not have one on their own.

This commit also deprecate +valid_actions+ in favor of just using +actions+ in
the LWRP DSL.
Use defined? to check for instance variables