PR to handle vm images for create vm scenarios

Changes in the PR include
1. Support for creating virtual machine using vm images.
2. Support for creating docker vm using vm images.
3. Image Show & Delete Image would now delete vm images.
4. Support for A8,A9 role size for create vm.
Logged an issue (https://github.com/MSOpenTech/azure-sdk-tools-xplat/issues/39) to track resolution of this comment in a subsequent sprint.