use 522-bit RSA keys in all unit tests (except one)

use 522-bit RSA keys in all unit tests (except one)

This reduces the total test time on my laptop from 400s to 283s.
* src/allmydata/test/test_system.py (SystemTest.test_mutable._test_debug):
  Remove assertion about container_size/data_size, this changes with keysize
  and was too variable anyways.
* src/allmydata/mutable/filenode.py (MutableFileNode.create): add keysize=
* src/allmydata/dirnode.py (NewDirectoryNode.create): same
* src/allmydata/client.py (Client.DEFAULT_MUTABLE_KEYSIZE): add default,
  this overrides the one in MutableFileNode