refactor(Makefile): better multi-node support

Our READMEs and Makefile really fall short for multi-node setups.
Specifically, we allude to the fact that DNS records will need to be
setup, but we don't provide additional info.

This commit updates the main README, contrib/ec2, and contrib/rackspace
with specific instructions for accessing the controller and router
for multi-node setups.

Additionally, the Makefile has been refactored so that there aren't
separate paths for 1-node and multi-node setups. Now we always start
the router, as we have provided instructions regarding DNS in the main README.
fix(systemd): handle starting containers that already exist

After a reboot, named containers for Deis components often
exist in a stopped state.  Prior to this commit,
Deis would fail to start the components due to a name conflict.
Now any existing named containers are removed in ExecStartPre
so ExecStart should always succeed.
Merge pull request #871 from deis/remove-on-start

fix(systemd): handle starting containers that already exist
docs(dns): move content from README.md into "Configure DNS" doc
Merge pull request #870 from deis/dns_readme

refactor(Makefile): better multi-node support
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