Update create VM example to use a supported OS
Update create VM example to use a supported OS

The Ubuntu 15.04 image was removed from the marketplace to updating the example to use 16.04.0-LTS latest.
v-iam/azure-sdk-for-python,Azure/azure-sdk-for-python,lmazuel/azure-sdk-for-python,SUSE/azure-sdk-for-python,Azure/azure-sdk-for-python,Azure/azure-sdk-for-python,AutorestCI/azure-sdk-for-python,Azure/azure-sdk-for-python,rjschwei/azure-sdk-for-python