[SPARK-18773][CORE] Make commons-crypto config translation consistent.

This change moves the logic that translates Spark configuration to
commons-crypto configuration to the network-common module. It also
extends TransportConf and ConfigProvider to provide the necessary
interfaces for the translation to work.

As part of the change, I removed SystemPropertyConfigProvider, which
was mostly used as an "empty config" in unit tests, and adjusted the
very few tests that required a specific config.

I also changed the config keys for AES encryption to live under the
"spark.network." namespace, which is more correct than their previous
names under "spark.authenticate.".

Tested via existing unit test.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #16200 from vanzin/SPARK-18773.