Revert "stack/monitor: cleanup deprecated (and confusing) config"

This reverts commit f34aeeab.

this had the side effect of installing `promise-check-free-disk-space` which
is still used - at least by caddy tests.

stack/monitor/buildout.hash.cfg

@@ -14,7 +14,7 @@
 # not need these here).
 [monitor2-template]
 filename = instance-monitor.cfg.jinja2.in
-md5sum = 3b0d2c8491f4cd77b4b48157739ef4a2
+md5sum = d4185c191e8b9df20e1f98cd8c556b1d
 
 [monitor-httpd-conf]
 _update_hash_filename_ = templates/monitor-httpd.conf.in

stack/monitor/instance-monitor.cfg.jinja2.in

@@ -97,6 +97,7 @@ parameter-file-path = ${monitor-instance-parameter:configuration-file-path}
 parameter-list =
   raw monitor-user ${monitor-instance-parameter:username}
   htpasswd monitor-password ${httpd-monitor-htpasswd:password-file} ${monitor-instance-parameter:username} ${httpd-monitor-htpasswd:htpasswd-path}
+  file min-free-disk-MB ${promise-check-free-disk-space:config-threshold-file}
   ${monitor-instance-parameter:instance-configuration}
 # htpasswd entry:  htpasswd key password-file username htpasswd-file
 
@@ -395,6 +396,7 @@ config-monitor-url = ${monitor-instance-parameter:monitor-base-url}
 module = check_free_disk_space
 name = check-free-disk-space.py
 config-collectordb = ${monitor-instance-parameter:collector-db}
+config-threshold-file = ${directory:etc}/min-free-disk-size
 
 [monitor-base]
 # create dependencies between required monitor parts