Fix|change system.repo update logic

   - Add possibility to remove prereq. packages installation BEFORE
     * Crucial logic violation - if we don't have any repo\
       have them configured in wrong way - stage will always fail.
     * install prereq. packages after all - sounds stupid, but correct.
     * By default - it will still try to install prereq. We don't want to
       broke OLD logic.See readme, how-to overide such behaviour.
   - don't update cache per-repo - it's simply useless and may fail due p1.
     Run update only once - after all repos configured\reconfigured
   - Add new option at system:refresh_repos_meta - for case, when update
     should not be run in any case. By default - true.
   - remove 99proxies-salt-{{ name }} along with disabled repo
   - fix duplicate 'clean_file' option

Closes-Bug: PROD-15992 (PROD:15992)

Change-Id: I4b312f82f65be80e7726f62482978f68c25746a3
diff --git a/README.rst b/README.rst
index 5fdc1fa..86f870f 100644
--- a/README.rst
+++ b/README.rst
@@ -711,6 +711,25 @@
                priority: 900
                package: '*'
 
+.. note:: For old ubuntu releases (<xenial)
+          extra packages for apt transport, like ``apt-transport-https``
+          may be required to be installed manually.
+          (Chicken-eggs problem: we need to install packages to
+          reach repo from where they should be installed)
+          Otherwise, you still can try 'fortune' and install prereq.packages before
+          any repo configuration, using list of requires in map.jinja.
+
+
+Disabling any prerequirment packages installation:
+You can simply drop any package pre-installation (before system.linux.repo
+will be processed) via cluster lvl:
+
+.. code-block:: yaml
+
+   linux:
+     system:
+       pkgs: ~
+
 
 Package manager proxy setup globally:
 
@@ -779,6 +798,14 @@
       system:
         purge_repos: true
 
+Refresh repositories metada, after configuration:
+
+.. code-block:: yaml
+
+    linux:
+      system:
+        refresh_repos_meta: true
+
 Setup custom apt config options:
 
 .. code-block:: yaml