Change rollback behavior

[decoupca]

Based on this discussion.

Currently, calling rollback() on NXOS does nothing unless napalm is aware of making changes. However, in some cases it makes sense to call rollback() in a new napalm session.

Proposed behavior is that rollback() will become agnostic to any changes napalm is aware of, and will have the net effect of reverting to the most recent state. For platforms with a native rollback feature, this will revert to the most recent snapshot. Platforms that don't, such as IOS, will attempt to revert to the rollback state that napalm created in a previous session. If such a rollback state does not exist, an exception will raise.