Environment variables for application services

[falexandrou]

We need to add the ability for the engine to look up environment variables and set them up as parameters or secrets in application services.

An environment variable in a service should address the following:

• An environment variable with a hardcoded value (eg. PORT=4000)
• An environment variable that proxies an environment variable from the host OS (eg. PORT=$STACKMATE_PORT)
• An environment variable that is constructed based on values from other services (eg. DATABASE_URL=postgresql://<db user name>:<db password>@<db host name>/<db name>?schema=public

We need to:

• Create placeholders for services
• Expose the placeholders in JSON schema
• Add a custom validator that validates placeholders in the config file