Import user-data page from openstack-manuals

This imports the "provide-user-data-to-instances" page
from the old openstack-manuals user guide.

Since we don't have a glossary, the :term: link is removed
and replaced with just giving the glossary definition as
the first part of the doc.

Change-Id: Iae70d9b53d6cefb3bcb107fe68499cccb71fc15e
Partial-Bug: #1714017

doc/source/index.rst

@@ -288,6 +288,7 @@ looking parts of our architecture. These are collected below.
    user/quotas
    user/support-matrix
    user/upgrade
+   user/user-data
    user/vendordata
    user/wsgi
 

doc/source/user/user-data.rst

@@ -0,0 +1,22 @@
+==============================
+Provide user data to instances
+==============================
+
+*User data* is a blob of data that the user can specify when they launch an
+instance. The instance can access this data through the metadata service or
+config drive. Commonly used to pass a shell script that the instance runs on
+boot.
+
+For example, one application that uses user data is the
+`cloud-init <https://help.ubuntu.com/community/CloudInit>`__ system,
+which is an open-source package from Ubuntu that is available on various
+Linux distributions and which handles early initialization of a cloud
+instance.
+
+You can place user data in a local file and pass it through the
+``--user-data <user-data-file>`` parameter at instance creation.
+
+.. code-block:: console
+
+   $ openstack server create --image ubuntu-cloudimage --flavor 1 \
+     --user-data mydata.file VM_INSTANCE