Added optional region parameter.  Fail if you specify an ELB that doesn'...

...t exist.

Boto blindly assumes the us-east-1 region if you don't hardcode a
region in it's config, so you could end up attempting to modify ELB's
in one region from a totally different region.  If a region isn't
specified then default to the region that the module is being run
within rather than the default us-east-1 region since it's a pretty
safe assumption that you intend to work on the ELB's within your
current region.

Also throw an error if a specified ELB instance doesn't exist.  The old
behavior would be to silently succeed with changed=false, so if you had
so much as a typo in the name of your ELB (or were in the wrong region
like my initial testing) you wouldn't get a clear indication that a
problem had occurred.