Normalizing Compute Statuses

Previously, each model had inline strings for each status that a resource could be. This PR normalizes those onto a Server.prototype.STATUS object for use across providers, but also for making setWait functions behave more consistently.

For example:

```Javascript
server.setWait({ status: 'RUNNING' }, function(err, resource) { ... });
```
Now becomes:

```Javascript
server.setWait({ status: server.STATUS.running }, function(err, resource) { ... });
```