Fix Vagrant caching

As of today, vagrant stopped working, my best guess is due to bad
caching. Here's an excerpt from logs:

...
vagrant plugin install vagrant-libvirt
Installing the 'vagrant-libvirt' plugin. This can take a few minutes...
Building native extensions. This could take a while...
Installed the plugin 'vagrant-libvirt (0.12.1)'!
...
uname -s ; cat Vagrantfile.$DISTRO
Linux
...

Downloaded 481Mb in 4.096201s.
Cache hit for vagrant-8be35383dc00f23d080ff00b2a724c938d650254861f26b67624c28e3fe5e6ae!
...
Vagrant failed to initialize at a very early stage:
The plugins failed to initialize correctly. This may be due to manual
modifications made within the Vagrant home directory.
...
Error message given during initialization: Unable to resolve dependency:
user requested 'vagrant-libvirt (= 0.12.0)'
...

The problem is, vagrant cache overwrites newer plugin with an older one.

Let's only cache the downloaded image.

Also, change the cache fingerprint script (remove "Linux").

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
(cherry picked from commit 650efb2c22)
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

.cirrus.yml

@@ -47,8 +47,8 @@ task:
     apt-get install -y --no-install-recommends libxslt-dev libxml2-dev libvirt-dev ruby-bundler ruby-dev zlib1g-dev
     vagrant plugin install vagrant-libvirt
   vagrant_cache:
-    fingerprint_script: uname -s ; cat Vagrantfile.$DISTRO
-    folder: /root/.vagrant.d
+    fingerprint_script: cat Vagrantfile.$DISTRO
+    folder: /root/.vagrant.d/boxes
   vagrant_up_script: |
     ln -sf Vagrantfile.$DISTRO Vagrantfile
     # Retry if it fails (download.fedoraproject.org returns 404 sometimes)