Open-source the Tectonic installer (#263)

* installer: open-source the Tectonic installer

* Jenkinsfile: integrate the Tectonic test/build/release suite

* installer: rename repository in imports, docs and scripts

* installer/server/terraform: copy templates from disk (prev. binassets)

Instead of extracting the TerraForm templates from binassets, find
them on disk (next to the installer's executable and upward in the tree
- or in cwd and upward in the tree) and copy them.

This enables us to bundle the TerraForm templates in the release tarball,
and let users customize them before running the GUI (or use them
directly using TerraForm).

* installer: refactor TF vendoring system to expose multiple providers

* installer/server/terraform/plugins: vendor local,template,aws,matchbox

* installer/vendor: add terraform-provider-matchbox and bump aws-sdk-go

* modules/{bootkube,tectonic}: update custom' providers name/inputs

According to the changes that were made for the upstreaming of these
providers.

* Makefile: drop the unused targets for building custom TerraForm bins

* Documentation: update documentation to use the installer for providers

.gitignore

@@ -1,5 +1,5 @@
-assets
-assets*.zip
+./assets
+./assets*.zip
 **/id_rsa*
 **/terraform.tfstate*
 .terraform