pyethapp does not respect the data_dir setting in config.yaml

[wangyifan]

Checklist

• [x ] installed pyethapp inside a virtualenv

System/environment

• OS: Ubuntu Ubuntu 16.04.3 LTS
• python version: Python 2.7.12
• pyethapp version: pyethapp v1.5.0a
• installation method (pip & pypi/git & setup.py): git
• pyethapp branch (if applicable): developer

Expected behaviour

1. Set data_dir in config.yaml
2. Expect pyethapp to respect the data_dir value set in config.yaml

Observed behaviour

1. Set data_dir in config.yaml
2. Run "pyethapp config" and it's still using the old data_dir value

Steps to reproduce

As explained in 'Expected behaviour' and 'Observed behaviour'

Currently it is only possible to use an alt data_dir by setting it in cli. IMHO, it would be better to have the app respect the data_dir value set in the config.yaml and make it possible to override it in cli.

I can prepare a fix for the above behavior if you guys are ok with it.