fix(contrib/coreos/user-data): prevent reboots; use $private_ipv4 for etcd/fleet

These commits:
* Move stopping the reboot services from the Vagrantfile to contrib/coreos/user-data where they can be enjoyed by all.
* Use `$private_ipv4` instead of `$public_ipv4` for all etcd/fleet communication. (See discussion in #868)
* Have EC2 respect DEIS_NUM_INSTANCES like every other provider does. 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.
* 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, #826 
replaces #860, #868