Change ubuntu base_image for vagrant to wily64

The ubuntu-bootstrap.sh script tries to install the packet
linux-image-generic-ls-wily when the kernel version is older than 4.2.
This fails when using the default base image ubuntu/vivid64. Therefore
this patch updates the base_image to ubuntu/wily64.

Change-Id: Ieaf2d35a9a4304de2d1d78d8245d115dcb32bb0e
Partial-Bug: #1547114

dev/vagrant/Vagrantfile

@@ -40,7 +40,7 @@ PROVIDER_DEFAULTS ||= {
       kolla_path: "/home/vagrant/kolla"
     },
     ubuntu: {
-      base_image: "ubuntu/vivid64",
+      base_image: "ubuntu/wily64",
       bridge_interface: "wlp3s0b1",
       vagrant_shared_folder: "/home/vagrant/sync",
       sync_method: "nfs",

dev/vagrant/Vagrantfile.custom.example

@@ -33,7 +33,7 @@
 #       kolla_path: "/home/vagrant/kolla"
 #     },
 #     ubuntu: {
-#       base_image: "ubuntu/vivid64",
+#       base_image: "ubuntu/wily64",
 #       bridge_interface: "wlp3s0b1",
 #       vagrant_shared_folder: "/home/vagrant/sync",
 #       sync_method: "nfs",