Refactor mutable parameter handling

The use of _mutable_parameters in the federation controllers was confusing.
The mutable parameters were the same each time check_immutable_params was
called, so there was no reason to allow passing in the mutable parameters.

In FederationProvider the mutable parameters was copied for no reason.

Similarly, filter_params is always used with the default keys so there's
no need to allow passing in the keys, and safer to not allow it.

bp identity-providers

Change-Id: I333fb23420282be350330dcb265f54da0fceac21
Merge "Refactor mutable parameter handling"