fix(contrib): check that coreos/user-data has a discovery_url set
chore(contrib): bump coreos to v343.0.0

```
core@deis-1 ~ $ docker version
Client version: 1.0.0
Client API version: 1.12
Go version (client): go1.2
Git commit (client): 63fe64c
Server version: 1.0.0
Server API version: 1.12
Go version (server): go1.2
Git commit (server): 63fe64c
core@deis-1 ~ $ fleetctl --version
fleetctl version 0.4.0
core@deis-1 ~ $ etcdctl --version
etcdctl version 0.4.3
```
fix(scheduler): remove arping hack

This is no longer required for v1.0.0
chore(contrib): bump coreos to v349.0.0

Still waiting on Rackspace, as they're still on v338.0.0 as of
this commit.
chore(builder): bump to docker v1.0
Merge pull request #1105 from deis/check-discovery-url

fix(contrib): check that coreos/user-data has a discovery_url set
fix(builder): set docker version explicitly
Merge pull request #1169 from deis/coreos-343.0.0

chore(contrib): bump coreos to v349.0.0