Correctly set registry_pkg_manager

With https://review.openstack.org/#/c/597153/
registry_pkg_manager was set to dnf for Centos7 as
well due to wrong string comparison, This patch convert
ansible_distribution_major_version to integer before
comparison and fixes the issue.

Change-Id: I603c1f932c63d07040e6aa24f1a75eab4e389738
Related-Blueprint: dnf-support

tasks/docker-update.yml

@@ -6,12 +6,12 @@
     registry_pkg_manager: yum
   when:
     - ansible_os_family == 'RedHat'
-    - ansible_distribution_major_version == 7
+    - ansible_distribution_major_version|int == 7
 
 - name: set package manager to dnf
   set_fact:
     registry_pkg_manager: dnf
-  when: (ansible_os_family == 'RedHat' and ansible_distribution_major_version > 7) or (ansible_distribution == 'Fedora')
+  when: (ansible_os_family == 'RedHat' and ansible_distribution_major_version|int > 7) or (ansible_distribution == 'Fedora')
 
 
 - name: can docker be updated