Add unit tests for environment variable injection in RecipientList endpoints

[n-jay]

Purpose

Add unit tests for variable injection capability in RecipientList endpoints.

Approach

Added new unit test to RecipientListEndpointSerializationTest class. It builds on the work amending AddressEndpointFactory class to extract environment variables via System.getenv() based on $SYSTEM prefix presence in the Synapse config XML URI elements.

Automation tests

• Unit tests Covers related code written in abstract class EndpointFactory.
• Integration tests N/A

Related PRs

Builds on the work done on PR https://github.com/apache/synapse/pull/55

Test environment

• Java version 1.8.0_141
• Ubuntu 20.04