Switch to using c3.large testing instances

This switches us over to the newer c3.large instance types in EC2. After some
testing I've found these boxes are much faster and can complete our tests at
a faster rate. Not only that for pricing they are priced at $0.105 versus
$0.130 (c1.medium) per instance hour.

And since c1.medium are now in the 'previous generation' bucket, best to switch
away anyway.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #1092 from kbarber/maint/stable/switch-to-c3-large-instances

Switch to using c3.large testing instances