gitlab: Add helper for setting up promises

A recipe could do

    [promise-<service>]
    <= promise-wrapper
    command-line = ...

and the wrapper will be put automatiaclly into etc/promise/<service>.

( for this to happen !py! magic is used again, like we did for logrotate
  and cron entries before )

/cc @kazuhiko, @jerome

software/gitlab/instance-gitlab.cfg.in

@@ -54,6 +54,12 @@ promise = ${:etc}/promise
 
 # 2. services
 
+# [promise-<something>] to generate promise wrapper <something>
+[promise-wrapper]
+recipe  = slapos.cookbook:wrapper
+wrapper-path = !py! '${directory:promise}/' + '${:_buildout_section_name_}'[8:]
+
+
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