Instance support

Support specifying `INSTANCE_NAMES=foo,bar` to deploy only to the listed instances.

Changes:
* Renamed script to `aws_opsworks_deploy` (totally open to other names, just tried to pick something more descriptive)
* Moved away from supporting optional arguments, instead optional items are via ENV-vars only
* `INSTANCE_NAMES=foo,bar` translates into a space-delimited list of instance IDs. If one isn't found, we `die` a horrible death.
* Disabled the defensive "set -e" since `grep` returns 1 when no match was found, and I am handling all those cases myself after the fact. With `set -e` I can't react and give a helpful error message.

I'll also add a couple questions/comments inline.
I felt this kept it simple, since supporting optional named parameters is really hairy as far as I can tell.