Support for using the existing data disk

[vishwabmc]

Today's operator always creates a new datadisk ( if asked for ) and this limits the number of deployments only to the new ones. However, there are usecases where we need to spin up the VSI attaching to an existing data disk. Without this support, the whole deployment need to be setup again.

For example : Let's say we are migrating from terraform deployment to operator deployment for Metaco containers, it relies on the stateful information that is present on the data disk. If it's not present, then it won't work per expectation.

@CarstenLeue @HenryWelborn