add github workflow

The GitHub job gets triggered only when the label
`ci-check/vagrant-setup` gets added to a PR.

The job runs on a self-hosted GitHub runner on Packet

.github/workflows/vagrant-packet.yaml

@@ -0,0 +1,17 @@
+name: Setup with Vagrant on Packet
+on:
+  push:
+  pull_request:
+    types: [labeled]
+
+jobs:
+  vagrant-setup:
+    if: contains(github.event.pull_request.labels.*.name, 'ci-check/vagrant-setup')
+    runs-on: self-hosted
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+    - name: Vagrant Test
+      run: |
+        export VAGRANT_DEFAULT_PROVIDER="virtualbox"
+        go test -v ./test/_vagrant