feat(contrib/ec2): use separate volume for /var/lib/docker

We've seen fewer performance issues when machines use a separate
volume for /var/lib/docker. This commit changes the AWS provision
script to automatically prepare and mount a separate volume for
Docker.

Note that we use the device name ``/dev/xvdf`` irrespective
of the instance virtualization type (PV or HVM). This seems to be
the way the kernel maps the block device, and isn't necessarily
unusual - see http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ebs-using-volumes.html

I manually tested this on both PV and HVM instances and had no issues.

There is no change in UX other than the new requirement of installing
the PyYAML python library.

closes #2793