[Starcluster] github changes

[Justin Riley]

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Hi All,

Just wanted to give those of you using the latest github code a heads up
on some recent changes:

1. AVAILABILITY_ZONE is now an optional setting in your cluster
templates. If you remove that option from your templates, StarCluster
will automatically determine, based on the EBS volumes specified if any,
which zone to launch the cluster in. If you do not have any volumes and
did not specify an AVAILABILITY_ZONE for your cluster template,
StarCluster will let Amazon choose the zone to launch the cluster in.
You can, of course, leave this setting provided it's valid.

2. The interface for the "start" action has changed slightly. I've
introduced the idea of a default template which is simply a cluster
template with DEFAULT=True. The template argument to the start command
is now an option: --cluster-template or -c for short. If this option is
not provided, the start command will attempt to use the default
template. So,

$ starcluster start mycluster

will launch a cluster tagged "mycluster" using the settings from the
cluster template that has DEFAULT=True. An error is raised if no default
template has been defined or if there is more than one default. To use
other templates besides the default:

$ starcluster start --cluster-template smallcluster mycluster

or

$ starcluster start -c smallcluster mycluster

For those that are interested, below is an explanation of why I chose to
make this change.

# Why?

The goal of this is to make the start command simpler for new users and
users that mostly only use one cluster template. Now these users can
ignore the template argument if they create a cluster template with
DEFAULT=True. This setting will be automatically configured in the
example config for new users.

# OK, so why not also make the tag name optional?
# Then it would just be "starcluster start" and you're off!

I kept the cluster tag as a required argument and not as an option that
defaults to anything because the start command costs money. Without any
required arguments it's too easy for a user to accidentally start a
cluster and be paying for it without knowing. I certainly don't want the
situation where a new user runs:

$ starcluster start

without knowing exactly what it will do and then have a cluster
magically starting and costing them money when they don't necessarily
intend to do so. Requiring this argument makes the user think about what
they're doing. Furthermore, if StarCluster were to just automagically
tag the cluster with some name there's a good chance the user might not
recognize the tag later on. The user's much more likely to recognize the
tag name later if they're required to choose the name in the first place.

~Justin
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