Support prerequisite resources for template validations

[khilscher]

CI test fails if template requires existing vNet name to be present. In some cases a template will reference an existing vNet and should fail if it doesn't exist (rather than create the vNet). e.g. a template which allows a Windows machine to join an existing AD-domain on an existing vNet. Currently the tests cannot accommodate this use case.

[sedouard]

Hey @khilscher. This validator was built off the assumption of not needing existing resources. We're aware of templates in the quick-starts template repo that require the need of existing resources but that hasn't been designed/built.

We're looking for someone from @singhkay's team to take over maintenance, deployment and new features of the server. In the meantime I won't be supporting new features, just bugs with the current server implementation.