Add config/docker command

[jasonhu5]

Docker credentials configurations are needed in scenarios not limited to AIDO challenges. Thus leaving the configuration in the AIDO book and with dts challenges command seem not sensible. This PR aims to rename dts challenges config to dts config docker, as well as leaving the possibility to keep all configurations under dts in dts config.

[jasonhu5]

Closing this PR now. Wait until daffy-staging is only ahead of daffy.

[jasonhu5]

For changes in config/docker, tested with following procedures:

• dts install config

• If arguments are incomplete: dts config docker

  usage: dts config docker [-h] [--docker-registry SERVER] --docker-username USERNAME --docker-password PASSWORD
  dts config docker: error: the following arguments are required: --docker-username, --docker-password

• dts config docker --docker-username jasonhu5 --docker-password abcde

• dts config docker --docker-username jason --docker-password passwd --docker-registry staging.dt.com