refactor(Makefile): use var for `fleetctl` command

This saves some repetition and allows us to test with different
versions of `fleetctl` by setting the environment variable
`FLEETCTL` to the appropriate binary and flags.
Merge pull request #997 from deis/makefile-fleet-var

refactor(Makefile): use var for `fleetctl` command
chore(contrib/*): update to CoreOS 317.0.0

Double-check the IDs:
EC2: https://coreos.com/docs/running-coreos/cloud-providers/ec2/
Rackspace: (coming soon!)