Implement list-service-group command
Merge branch 'docker-grouping-merge' of https://git-wip-us.apache.org/repos/asf/stratos into docker-grouping-merge

Conflicts:
	components/org.apache.stratos.cli/src/main/java/org/apache/stratos/cli/RestCommandLineService.java
Merge branch 'docker-grouping-merge' of https://git-wip-us.apache.org/repos/asf/stratos into docker-grouping-merge
Merge branch 'docker-grouping-merge' of https://git-wip-us.apache.org/repos/asf/stratos into docker-grouping-merge
Merge branch 'docker-grouping-merge' of https://git-wip-us.apache.org/repos/asf/stratos into docker-grouping-merge
Add list-service-group command in CLI
Refactor list-service-group to describe-service-group