How to pass Environment Variable in contexts yaml

[DeChrish]

We are using SauceLabs properties, and did not want to add the username and password in yaml file, instead add the environment variable.

Ex:
webdriver.type: remote
webdriver.url: $Sauce_SSO_URL
webdriver.cap.platform: WIN10
webdriver.cap.browserName: chrome
webdriver.cap.version: latest
webdriver.cap.extendedDebugging: true