monitor: support http-header-dict.
Introducing check-http-header-dict parameter to check HTTP header dict introduced in https://lab.nexedi.com/nexedi/surykatka/merge_requests/10 and https://lab.nexedi.com/nexedi/slapos.toolbox/merge_requests/84.
Introducing check-http-header-dict parameter to check HTTP header dict introduced in https://lab.nexedi.com/nexedi/surykatka/merge_requests/10 and https://lab.nexedi.com/nexedi/slapos.toolbox/merge_requests/84.