Support rollback for `config set`

[SteveL-MSFT]

Summary of the new feature / enhancement

Prior to set the current state via get is stored. If set fails, dsc can attempt a rollback to previous. It may make sense to have a --no-rollback switch for developers.

Proposed technical implementation details (optional)

If an application wants multiple levels of rollback, it's up to that application to store history. If resources in the rollback state support _clobber, then those should be explicitly set to true.