Set a default value for an environment variable if it does not exist

Substitute the value of other environment variables into the named one.

For example:

process.env.MAJOR_VERSION = "0";
                                    process.env.MINOR_VERSION = "3";
                                    EnvironmentVariables.substitute('api/.');
                                    

... will return 'api/0.3';

Note that if this is done for multiple environment variables, the order in which they are done will affect the result. It is the responsibility of the caller to this function to ensure that the values to be substituted in are already fully resolved: this function is not recursive. Therefore, cycles are also not allowed. Any of these will result in indeterminate results, including errors.

Test that an environment variable exists

Enforce that an environment variable exists