Switch to using Go modules.
This migrates our vendor.json to use Go 1.11's modules system, and
replaces the vendor folder with the output of go mod vendor.
The vendored code should remain basically the same; I believe some
tree shaking of packages and support scripts/licenses/READMEs/etc.
happened.
This also fixes Travis and our Makefile to no longer use govendor.
This was done as its own resource as suggested in slack, since we don't have the option of making all fields Computed in google_compute_instance. There's precedent in the aws provider for this sort of thing (see ami_copy, ami_from_instance).
When I started working on this I assumed I could do it in the compute_instance resource and so I went ahead and reordered the schema to make it easier to work with in the future. Now it's not quite relevant, but I left it in as its own commit that can be looked at separately from the other changes.
Fixes#1582.
