Policies were always applied to target `all` aspects and
there was no way to have them target only e.g. exchanges.
This can be important though, see [1].
This change enables the use of the apply_to parmeter via the existing
variables while maintaining `all` to be the default.
[1] https://www.rabbitmq.com/docs/parameters#how-policies-work
Change-Id: I61d68c630599b8ef2382663e8a37fdc456435c11