build_config(env): not escape space accordingly

[aarnphm]

Describe the bug

Currently if users pass in space environment variables BuildConfig wouldn't be able to process it accordingly

service: "service.py:svc"
env:
  my space: 1
  ENV: hello world

To Reproduce

Expected behavior

Screenshots/Logs

Environment:

• OS: [e.g. MacOS 10.14.3]
• Python Version [e.g. Python 3.7.1]
• BentoML Version [e.g. BentoML-0.8.6]

Additional context