Add Tests For Environment Variable Override

[michaelestrin]

Related edgex-go issue: edgexfoundry/edgex-go#1450
Related edgex-go PR: edgexfoundry/edgex-go#1461

Add tests for newly added edgex-go environment variable override support.

Core services were modified to:

• Support environment variable overrides for config-seed to the configuration.toml content it uploads into registry. An environment variable can be service-specific or generic.

  A service-specific environment variable override would include the targeted service's name and would only affect configuration for that service; i.e. core-metadata_Clients_logging_Host=<someAddr>.

  A generic environment variable override would not include a service name and would affect configuration across all services config-seed handles; i.e. Registry_Host=<someAddr>.

• Support environment variable override of a core service's RegistryInfo fields (i.e. host, port, and type) to allow a containerized service instance to be pointed at an alternate registry location.

[jinlinGuan]

Move this issue to edgexfoundry/edgex-taf#18

[michaelestrin]

Why was this moved and closed? I would like to have it reopened here as it's an outstanding issue with the existing blackbox tests (which is still our testing standard). @cloudxxx8 @jinlinGuan

[michaelestrin]

This issue has a dependency on edgexfoundry/edgex-go#2236