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

Avoids intermittent startup errors when instances not yet ready.

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>`