health-check: fix when running after upgrade

The preprod upgrade create a review branch in /root/config, and the config_reset
task doesn't switch back to master. This caused the .git/refs/heads/master check
in config-update to not work properly.

Change-Id: Ia30b0e94a2cbe52b1eb25c2687ffc0ad527d42d2

Author: TristanCacqueray

Diff for: playbooks/config_reset.yaml

@@ -6,5 +6,6 @@
       command: chdir=/root/config {{ item }}
       with_items:
         - git fetch --all
+        - git checkout master
         - git reset --hard origin/master --
         - git clean -f -d