Example app - use env variables for collector url and app id

This MR is to use environment variables `PA_COLLECTOR_URL`, `PA_APPLICATION_ID` to define snowplow collector URL and application id in example application.

These changes are needed for the future tests. The plan is to build docker image of example application and then start it within a test with snowplow collector URL and application id specific to that test, this can be achieved by passing corresponding environment variables to a command that will start docker container for example app.