fix(contrib/ec2): use contrib/coreos/user-data

Previously, contrib/ec2 used its own stripped-down
user-data file. We have customizations in contrib/coreos/user-data,
and we want all providers using the same user-data file.

This commit adds a Python script which will dynamically generate
the appropriate JSON payload from contrib/coreos/user-data when
running the provision-ec2-cluster script. While ideally this would
be achieved without requiring an external scripting language,
this is very easy to achieve in Python, and we already require Python
as a dependency (since the AWS CLI is a Python package).

closes #858