Fix ubuntu source_ami_filter examples

Canonical publishes stable AMIs at ubuntu/images/ and unstable AMIs at
ubuntu/images-testing/. Combined with most_recent: true, the old name
filter bounces you between stable and unstable.

website/source/docs/builders/amazon-chroot.html.md

@@ -269,7 +269,7 @@ each category, the available configuration keys are alphabetized.
     "source_ami_filter": {
       "filters": {
         "virtualization-type": "hvm",
-        "name": "*ubuntu-xenial-16.04-amd64-server-*",
+        "name": "ubuntu/images/*ubuntu-xenial-16.04-amd64-server-*",
         "root-device-type": "ebs"
       },
       "owners": ["099720109477"],

website/source/docs/builders/amazon-ebs.html.md

@@ -264,7 +264,7 @@ builder.
       "source_ami_filter": {
         "filters": {
           "virtualization-type": "hvm",
-          "name": "*ubuntu-xenial-16.04-amd64-server-*",
+          "name": "ubuntu/images/*ubuntu-xenial-16.04-amd64-server-*",
           "root-device-type": "ebs"
         },
         "owners": ["099720109477"],

website/source/docs/builders/amazon-ebssurrogate.html.md

@@ -257,7 +257,7 @@ builder.
       "source_ami_filter": {
         "filters": {
           "virtualization-type": "hvm",
-          "name": "*ubuntu-xenial-16.04-amd64-server-*",
+          "name": "ubuntu/images/*ubuntu-xenial-16.04-amd64-server-*",
           "root-device-type": "ebs"
         },
         "owners": ["099720109477"],

website/source/docs/builders/amazon-ebsvolume.html.md

@@ -170,7 +170,7 @@ builder.
       "source_ami_filter": {
         "filters": {
           "virtualization-type": "hvm",
-          "name": "*ubuntu-xenial-16.04-amd64-server-*",
+          "name": "ubuntu/images/*ubuntu-xenial-16.04-amd64-server-*",
           "root-device-type": "ebs"
         },
         "owners": ["099720109477"],

website/source/docs/builders/amazon-instance.html.md

@@ -262,7 +262,7 @@ builder.
       "source_ami_filter": {
         "filters": {
           "virtualization-type": "hvm",
-          "name": "*ubuntu-xenial-16.04-amd64-server-*",
+          "name": "ubuntu/images/*ubuntu-xenial-16.04-amd64-server-*",
           "root-device-type": "ebs"
         },
         "owners": ["099720109477"],

website/source/intro/getting-started/build-image.html.md

@@ -58,7 +58,7 @@ briefly. Create a file `example.json` and fill it with the following contents:
     "source_ami_filter": {
       "filters": {
       "virtualization-type": "hvm",
-      "name": "*ubuntu-xenial-16.04-amd64-server-*",
+      "name": "ubuntu/images/*ubuntu-xenial-16.04-amd64-server-*",
       "root-device-type": "ebs"
       },
       "owners": ["099720109477"],