[r=jameinel],[bug=1300321] cloudinit: always install curl
[r=jameinel],[bug=1300321] cloudinit: always install curl

Our cloud images always come with curl installed, but manual instances may
not (bug #1300321). It doesn't hurt to install a package that is already
there, so this is a trivial patch to just depend on having curl installed.

https://codereview.appspot.com/83250044/
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