suggest: support load config from .env

[Weihong Xu]

Since jjb has already supported to load default config from environment variable, it would be nice to support to load config from .env as well. 

https://review.opendev.org/c/jjb/jenkins-job-builder/+/858259

[link89]

The reason I hope to support .env file is it allow me to set configuration at the project level.  When I run jjb command in the project root folder and its sub folders, it will be use automatically.