Explicitly define metadata provider for Vultr

Vultr provides an API that looks a lot like the AWS api, resulting in
the AWS provider succeeding, but missing certain metadata parts that one
would expect to work out of the box on Vultr, such as SSH PubKey
fetching.

Signed-off-by: Sachi King <nakato@nakato.io>

examples/vultr.yml

@@ -14,6 +14,7 @@ onboot:
     command: ["/sbin/dhcpcd", "--nobackground", "-f", "/dhcpcd.conf", "-1"]
   - name: metadata
     image: linuxkit/metadata:v0.7
+    command: ["/usr/bin/metadata", "vultr"]
 services:
   - name: getty
     image: linuxkit/getty:v0.7