do not limit ec2 region to static list of regions

I'm not sure this should be Wishlist..

Every time Amazon adds a new region this will *appear* as if Juju can't
talk to it.

We of course know that we can change the ec2-url to point at whichever
region we want, but that will be too late, as some users will just assume
it just won't work. :-/

Excerpts from Kapil Thangavelu's message of Fri Jan 27 03:15:16 UTC 2012:
> ** Changed in: juju
> Importance: Low => Wishlist
>
> --
> You received this bug notification because you are subscribed to the bug
> report.
> https://bugs.launchpad.net/bugs/893176
>
> Title:
> do not limit ec2 region to static list of regions
>
> Status in juju:
> New
>
> Bug description:
> Amazon opens new regions regularly and the static list becomes a
> limiting factor immediately. Remove the static list and instead just
> validate it for proper format. The format would just be a valid DNS
> hostname, since it is just prepended to amazonaws.com.
>
> The value could be validated in a second pass once an EC2 connection
> has been established using the AWS call DescribeRegions , but that
> call does not currently exist in txaws.
>
> Note that this is a Low priority, as there is a workaround, which is
> to just set the ec2-uri to the appropriate endpoint for the desired
> region.
>
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