Packer mirror improvments

- Change the default country to US
- Add mirror/* settings to set up archive.ubuntu.com as a default
  mirror. This allows to use fastest mirror rather than sticking to CA
  mirror
- Remove choose-mirror-bin as d-i mirror/http/proxy does the same

Change-Id: I5e56adf6c0d3452dcd57c4fd79feabf08cf7774a

utils/packer/http/ubuntu-16.04/preseed.cfg

@@ -1,4 +1,3 @@
-choose-mirror-bin mirror/http/proxy string
 d-i debian-installer/locale string en_US
 d-i debian-installer/language string en
 d-i debian-installer/country string US
@@ -34,3 +33,8 @@ d-i user-setup/allow-password-weak boolean true
 d-i user-setup/encrypt-home boolean false
 tasksel tasksel/first multiselect standard, ubuntu-server
 cloud-init cloud-init/datasources multiselect NoCloud, None
+d-i mirror/country string manual
+d-i mirror/http/hostname string archive.ubuntu.com
+d-i mirror/http/mirror select archive.ubuntu.com
+d-i mirror/http/directory string /ubuntu
+d-i mirror/http/proxy string

utils/packer/ubuntu.json

@@ -63,8 +63,8 @@
         "auto=true ",
         "url=http://{{.HTTPIP}}:{{.HTTPPort}}//{{ user `preseed_file_name` }} ",
         "language=en ",
-        "country=CA ",
-        "locale=en_CA.UTF-8 ",
+        "country=US ",
+        "locale=en_US.UTF-8 ",
         "hostname=ubuntu1604 ",
         "domain=ubuntu1604 ",
         "console-setup/ask_detect=false ",
@@ -112,8 +112,8 @@
         "auto=true ",
         "url=http://{{.HTTPIP}}:{{.HTTPPort}}//{{ user `preseed_file_name` }} ",
         "language=en ",
-        "country=CA ",
-        "locale=en_CA.UTF-8 ",
+        "country=US ",
+        "locale=en_US.UTF-8 ",
         "hostname=ubuntu1604 ",
         "domain=ubuntu1604 ",
         "console-setup/ask_detect=false ",