feat(etcd-discovery): Add support for etcd discovery URLs

A user can submit a bash script in their EC2 user-data that
will be sourced by coreos-c10n. If an environment variable
ETCD_DISCOVERY_URL was exported, it will be used to
bootstrap the local etcd service instead of the actual
c10n service.
fix(c10n): clean up temp file
fix(c10n): execute rather than source script-like user-data
fix(c10n): preserve initial user-data as c10n token
fix(c10n): do not exit on successful ssh-key injection

This exit should not have been added in the first place.
The user-data can still be used as a c10n token, even
if an SSH key was found after following the user-data
as a URL.
fix(c10n): use update-ssh-keys
fix(c10n): always quote $USER_DATA
fix(c10n): head the tmpfile rather than piping $USER_DATA
Merge pull request #43 from bcwaldon/etcd-disco

feat(etcd-discovery): Add support for etcd discovery URLs