Optimize Vagrant setup

- define grains to match optimized gvasalt repository
- define minion configuration in separate file
- drop custome salt/bootstrap.sh
- adapt Vagrantfile to use the custom grains file
- increase memory size of created VM to allow the salt provisioning to
  succeed

salt/minion

@@ -0,0 +1,11 @@
+file_client: local
+
+file_roots:
+  base:
+    - /srv/salt/
+
+pillar_roots:
+  base:
+    - /srv/pillar
+
+log_file: file:///dev/log