playbook: upgrade slapcache tools if needed before launch upgrader playbook

Some servers failed to run upgrader because slapos.libnetworkcache was old and not compatible anymore. So playbook couldn't be downloaded.
We check if slapos.libnetworkcache and slapcache need to be upgraded before run upgrader role.

/reviewed-on !65

playbook/roles/slapcache/tasks/main.yml

+  # easy_install module for ansible version on debian 8 cannot upgrade package
+  - name: Install/Upgrade latest slapos.libnetworkcache 
+    shell: easy_install --upgrade slapos.libnetworkcache
+
+  - name: Instal/Upgrade latest slapcache
+    shell: easy_install --upgrade slapcache

playbook/roles/upgrader/meta/main.yml

+---
+dependencies:
+  - { role: slapcache }