Allow the user to override the npm start command when using Node Builder Image

Description

As a user, I want to override the "npm run start" command when using Node Builder Image.

This works already when the user define the NPM_RUN environment variable.

Acceptance Criteria

1. When importing an app, I should be able to easily provide the "npm run start" command. Earlier mentioned "npm run build" command could not be overridden.

Additional Details:

Possible issue / Implementation options (maybe timeboxed??):

1. Check first if it's possible to sync the new field with the Deployment environment variables.
2. Create an extension which renders a (form) component based on the Image Stream (name and version) between the Image stream selection and the General section

To override the start command the Environment variable NPM_RUN should be set. See https://github.com/sclorg/s2i-nodejs-container/blob/master/12/README.md#environment-variables-for-source-to-image

Desired UX experience: The (only one!!) new field should be displayed below the image stream description. The field should override the environment variable(s).