Add a key() method for RuntimeStack

Currently, the RuntimeStack has a toString() method which looks like to show the displayname of the Runtime.
https://github.com/Azure/azure-libraries-for-java/blob/3685f520d45461cbfe8e131e74336cc5420f1fcb/azure-mgmt-appservice/src/main/java/com/microsoft/azure/management/appservice/RuntimeStack.java

Could it be possible if we add another method(saying: `.key()`) to show the key name of the runtime?

And the key should follow the docker convention, like: `tomcat:8.5-jre8`