Check that environment variables exist

[moritzraho]

Description

Related Issue

Motivation and Context

An example: if devs forget to specify APIHOST, the app is build with wrong action URLs. Throwing an error on missing APIHOST solves the problem.

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have signed the Adobe Open Source CLA.
• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[Himavanth]

Asking for my knowledge. Do we have a standard for openwhisk env variables in CLI ? WHISK vs WSK vs OW ?

[moritzraho]

@Himavanth the advantage of WHISK_* is that it can be picked up by wsk/aio runtime CLIs directly