Wait for instances to move out of pending state before adding name tags.

John;
This small fix avoids intermittent startup errors when instances not yet ready. I would ocassionally see annoying cases where node startup errored out with:
```
ERROR:gc3.elasticluster:could not start node `compute002` for reason `EC2ResponseError: 400 Bad Request
<?xml version="1.0" encoding="UTF-8"?>
<Response><Errors><Error><Code>InvalidInstanceID.NotFound</Code><Message>The instance ID 'i-0df866e1' does not exist</Message></Error></Errors><RequestID>58c878c1-2c59-484c-835c-44b9176fae77</RequestID></Response>`
```
resulting in an unusable cluster. Waiting for instances to move out of the pending state avoids these and creates a cleaner startup.