fix(contrib/coreos/user-data): handle reboots in shared user-data; have ec2 use it

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.

These commits:
* Move stopping the reboot services from the Vagrantfile to contrib/coreos/user-data where they can be enjoyed by all.
* Introduce 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, #859 
replaces #860