Support for passing environment variables to the runtime environment for an application.

[markpollack]

The user should be able to specify environment variables to the environment when running an application.

For example, to set SPRING_CLOUD_CONFIG_URI=http://localhost:8888

This request originated in https://github.com/spring-cloud/spring-cloud-dataflow/pull/414 but was before switching over to AppDeployer.

The current AppDeployer has a Map<String, String> environmentProperties; property which will be renamed to be deploymentProperties via.

There should be a new property, Map<String, String> environmentVariables to capture these environment variables in AppDeployer.

In turn there can be default configuration properties for Data Flow so that a common set of env-vars can be set for use across all deployments and also support to provide them as deployment properties per application. See https://github.com/spring-cloud/spring-cloud-dataflow/issues/661 and https://github.com/spring-cloud/spring-cloud-dataflow/issues/662