Merge pull request #2 from chapmanb/bcbio-pgs

Support for specifying placement groups, enabling low-latency networking between cluster instances
Support Amazon AMIs: identify as CentOS and required tweaks

- Amazon AMIs are based on RedHat and a bit more minimal, so it requires
  a few additional packages installed in addition to the current CentOS
  support.
- Amazon AMIs use /dev/xvda for the root volume, so this generalizes the
  root volume customizations from gc3-uzh-ch/elasticluster#133
better naming
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>`
```
Merge with jwm/bcbio