[config] use derived default when possible
[config] use derived default when possible

SecureConfig can derive default at no loss

Closes: #3690
Approved by: joshlind

aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra