fix(contrib): #859 remove update manager stop commands from Vagrantfile
fix(user-data): use $private_ipv4 for etcd/fleet

All etcd/fleet communication should be private, so have etcd/fleet
only listen on private IPs. Remote fleetctl still works, as it uses
an SSH tunnel. There's really no reason to ever use $public_ipv4.
fix(contrib/ec2): use shared 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.

To do this, we 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).

Also, have EC2 respect DEIS_NUM_INSTANCES like every other provider does.

closes #858, #826
Merge pull request #869 from deis/contrib_use_user_data

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