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, 32 insertions, 31 deletions

diff --git a/doc/examples/cloud-config-chef-oneiric.txt b/doc/examples/cloud-config-chef-oneiric.txt
index ab2e2a33..241fbf9b 100644
--- a/doc/examples/cloud-config-chef-oneiric.txt
+++ b/doc/examples/cloud-config-chef-oneiric.txt
@@ -13,38 +13,39 @@
 # Key from http://apt.opscode.com/packages@opscode.com.gpg.key
 apt:
   sources:
-   - source: "deb http://apt.opscode.com/ $RELEASE-0.10 main"
-     key: |
-       -----BEGIN PGP PUBLIC KEY BLOCK-----
-       Version: GnuPG v1.4.9 (GNU/Linux)
+     source1: 
+        source: "deb http://apt.opscode.com/ $RELEASE-0.10 main"
+        key: |
+         -----BEGIN PGP PUBLIC KEY BLOCK-----
+         Version: GnuPG v1.4.9 (GNU/Linux)
 
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-       DAAKCRApQKupg++CauISAJ9CxYPOKhOxalBnVTLeNUkAHGg2gACeIsbobtaD4ZHG
-       0GLl8EkfA8uhluM=
-       =zKAm
-       -----END PGP PUBLIC KEY BLOCK-----
+         mQGiBEppC7QRBADfsOkZU6KZK+YmKw4wev5mjKJEkVGlus+NxW8wItX5sGa6kdUu
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+         0GLl8EkfA8uhluM=
+         =zKAm
+         -----END PGP PUBLIC KEY BLOCK-----
 
 chef: