This reworks the template lifecycle a bit such that we get nicer diff
behavior.
First, we tick ForceNew on for both filename and vars, so that the diff
indicates that the template will be "replaced" on change. This is mostly
cosmetic, but it also tracks conceptually with the fact that the
identifier we use is a hash of the contents, so any change essentially
makes a "new resource".
Second, we change the Exists implementation to only return `false` when
there has been a change in the rendered template. This lets descendent
resources see the computed value changing so that they'll properly
trigger in the plan.
Fixes#1898
Refs #1866 (but does not fix, there's another deeper issue there)
a96a3372c6