Make Pinning compatible with Pike repos

The Release file structure was suddenly changed starting from Pike
repo. And current pinning pattern doesn't work for >= Pike.
As it was suggested by infra team we should update it.

Change-Id: I7952ff85cfb6fd2bb6158c520f1ba3d5f05f9abc
Related-Prod: PROD-16408
diff --git a/linux/system/repo/mcp/openstack.yml b/linux/system/repo/mcp/openstack.yml
index baa363a..465b75d 100644
--- a/linux/system/repo/mcp/openstack.yml
+++ b/linux/system/repo/mcp/openstack.yml
@@ -10,7 +10,7 @@
           architectures: amd64
           key_url: "http://mirror.fuel-infra.org/mcp-repos/${_param:openstack_version}/${_param:linux_system_codename}/archive-mcp${_param:openstack_version}.key"
           pin:
-          - pin: 'release a=${_param:openstack_version}'
+          - pin: 'release l=${_param:openstack_version}'
             priority: 1100
             package: '*'
         mk_openstack:
@@ -18,6 +18,6 @@
           architectures: amd64
           key_url: "http://apt.mirantis.com/public.gpg"
           pin:
-          - pin: 'release a=${_param:linux_system_repo_mk_openstack_version}'
+          - pin: 'release l=${_param:linux_system_repo_mk_openstack_version}'
             priority: 1100
             package: '*'
diff --git a/linux/system/repo_local/mcp/openstack.yml b/linux/system/repo_local/mcp/openstack.yml
index a25cc57..08f1e67 100644
--- a/linux/system/repo_local/mcp/openstack.yml
+++ b/linux/system/repo_local/mcp/openstack.yml
@@ -7,6 +7,6 @@
           architectures: amd64
           key_url: "http://${_param:local_repo_url}/public.gpg"
           pin:
-          - pin: 'release a=${_param:openstack_version}'
+          - pin: 'release l=${_param:openstack_version}'
             priority: 1100
             package: '*'