Use 'bash' in the keepalived check script

Use bash instead of sh to avoid the error "shopt: not found"

Change-Id: Ib089affa229531cd72f6853105d74b446687ae86
Story: 2008437
Task: 41399
(cherry picked from commit 1a86e454e7)

octavia/amphorae/backends/agent/api_server/templates/keepalived_check_script.conf.j2

@@ -23,7 +23,7 @@ status=0
 for file in {{ check_scripts_dir }}/*
 do
   echo "Running check script: " $file
-  sh $file
+  bash $file
   status=$(( $status + $? ))
 done
 exit $status