Transition from internal.py defaults in config.py to schema defaults.

[bschroeter]

We currently rely on internal.py defaults to specify optional keys in the configuration through a manually-set config dictionary:

benchcab/src/benchcab/config.py

Line 74 in a44e986

def read_optional_key(config: dict):

We can get around doing this by setting the defaults via the Cerberus config schema.

[SeanBryan51]

Duplicate issue: #284