Add ability to secure all available repos

Add parameter which allows to secure all repos available or directly
specify which should be existed. Also precedence repo parameters over
common_repo_secured saved.

Change-Id: Ic8d72bcd4457dfce94088ff6bd0a1d4dc23d318e
Related-Prod: PROD-23434
diff --git a/README.rst b/README.rst
index ebccec8..7e580f3 100644
--- a/README.rst
+++ b/README.rst
@@ -1218,6 +1218,49 @@
             secure: true
             url: example2.org/ubuntu
 
+Also it is possible to specify list of repos, which should be secured
+within ``common_repo_secured`` block and without changing current
+existing repo source parameter:
+
+.. code-block:: yaml
+
+    linux:
+      system:
+        ...
+        common_repo_secured:
+          user: foo
+          password: bar
+          secured_repos: [ 'test1', 'test2' ]
+        repo:
+          test1:
+            ...
+          test2:
+            ...
+          test3:
+            ...
+
+Repos ``test1, test2`` will be secured. In case if you want secure all
+available repos use ``secured_repos: [ 'all' ]``. But repo parameters have
+precedence over parameters from ``common_repo_secured``. In next case:
+
+    linux:
+      system:
+        ...
+        common_repo_secured:
+          user: foo
+          password: bar
+          secured_repos: [ 'all' ]
+        repo:
+          test1:
+            ...
+          test2:
+            ...
+          test3:
+            secure: False
+            ...
+
+Repo ``test3`` will not be secured.
+
 Remove all repositories:
 
 .. code-block:: yaml