Add database pod and service for ruby sample app demo

- [x] update ruby-sample-app image with MySQL connection for Sinatra app 
- [x] change docker image URL with OpenShift official image for ruby-sample-app - https://github.com/openshift/ruby-hello-world/pull/5
I tried to cover all possibilities:

1. Use expression generators -> for Admin password/username
2. Specify variable value -> MySQL password
3. Use the MySQL default environment variable as the database name, to show how containers share the same environment variables when they are linked.