Add schema to apt configure config (#357)

Create a schema object for the `apt_configure` module and
validate this schema in the `handle`  function of the module.

There are some considerations regarding this PR:

* The `primary` and `security` keys have the exact same properties. I
  tried to eliminate this redundancy by moving their properties to a
  common place and then just referencing it for both security and
  primary. Similar to what is documented here:
    https://json-schema.org/understanding-json-schema/structuring.html
  under the `Reuse` paragraph.  However, this approach does not work,
  because the `#` pointer goes to the beginning of the file, which is
  a python module instead of a json file, not allowing the pointer to
  find the correct definition. What I did was to create a separate dict
  for the mirror config and reuse it for primary and security, but
  maybe there are better approaches to do that.
* There was no documentation for the config `debconf_selections`. I
  tried to infer what it supposed to do by looking at the code and the
  `debconf-set-selections`  manpage, but my description may not be
  accurate or complete.
* Add a _parse_description function to schema.py to render multi-line
  preformatted content instead of squashing all whitespace

LP: #1858884

1 files changed, 1 insertions, 1 deletions

diff --git a/doc/examples/cloud-config-apt.txt b/doc/examples/cloud-config-apt.txt
index 5a56cd1b..004894b7 100644
--- a/doc/examples/cloud-config-apt.txt
+++ b/doc/examples/cloud-config-apt.txt
@@ -142,7 +142,7 @@ apt:
       # as above, allowing to have one config for different per arch mirrors
   # security is optional, if not defined it is set to the same value as primary
   security:
-    uri: http://security.ubuntu.com/ubuntu
+    - uri: http://security.ubuntu.com/ubuntu
   # If search_dns is set for security the searched pattern is:
   #   <distro>-security-mirror