pkg: Add a generic metadata package

This package handles meta and user data for different cloud
and other platforms. It should be easy to extend to new
platforms.

Currently, it handles GCP metadata and a simple CDROM userdata
provider.

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>

docs/metadata.md

@@ -0,0 +1,70 @@
+# Metadata and Userdata handling
+
+Most providers offer a mechanism to provide a OS with some additional
+metadata as well as custom userdata. `Metadata` in this context is
+fixed information provided by the provider (e.g. the host
+name). `Userdata` is completely custom data which a user can supply to
+the instance.
+
+The [metadata package](../pkg/metadata/) handles both metadata and
+userdata for a number of providers (see below).  It abstracts over the
+provider differences by exposing both metadata and userdata in a
+directory hierarchy under `/var/config`.  For example, sshd config
+files from the metadata are placed under `/var/config/ssh`.
+
+Userdata is assumed to be a single string and the contents will be
+stored under `/var/config/userdata`.  If userdata is a json file, the
+contents will be further processed, where different keys cause
+directories to be created and the directories are populated with files. Foer example, the following userdata file:
+```
+{
+    "ssh" : {
+        "sshd_config" : {
+            "perm" : "0600",
+            "content": "PermitRootLogin yes\nPasswordAuthentication no"
+        }
+    },
+    "foo" : {
+        "bar" : {
+            "perm": "0644",
+            "content": "foobar"
+        },
+        "baz" : {
+            "perm": "0600",
+            "content": "bar"
+        }
+    }
+}
+```
+will generate the following files:
+```
+/var/config/ssh/sshd_config
+/var/config/foo/bar
+/var/config/foo/baz
+```
+
+This hierarchy can then be used by individual containers, who can bind
+mount the config sub-directory into their namespace where it is
+needed.
+
+
+# Providers
+
+Below is a list of supported providers and notes on what is supported. We will add more over time.
+
+
+## GCP
+
+GCP metadata is reached via a well known URL
+(`http://metadata.google.internal/`) and currently
+we extract the hostname and populate the
+`/var/config/ssh/authorized_keys` from metadata. In the future we'll
+add more complete SSH support.
+
+GCP userdata is extracted from `/computeMetadata/v1/instance/attributes/userdata`.
+
+
+## HyperKit
+
+HyperKit does not support metadata and userdata is passed in as a single file via a ISO9660 image.
+