Document usage of environment variables in Pysys test framework

[haraldmeyer]

Related to here #5, the Pysys testing now supports the usage of environment variables. Two issues:

• the need to configure environment variables is only documented in the reference (https://open-source.softwareag.com/apama-eplapps-tools/autodocgen/apamax.eplapplications.basetest.html) but not in the documentation when creating ones own test case.
• the framework is not using the same variables that go-c8y-cli is using making it difficult to use it session management functionality. Namely go-c8y-cli is not providing CUMULOCITY_SERVER_URL

[skom-c8y]

This is tracked internally via PAB-4502