mgr/cephadm: add node-proxy endpoints to the mgr

mgr/cephadm: add node-proxy endpoints to the mgr

This adds 2 endpoints to the existing http agent endpoint:

- '/node_proxy/idrac': support POST requests only although this endpoint
  is intended for fetching the idrac credentials of a given node. As we pass
  sensitive details (ceph secret) I didn't want to pass it as a query parameter
  in the url. Passing it in a HTTP header is perhaps a better approach but we already
  do similar thing for endpoint '/data' (agent) so for consistency reason I stick to
  that.

- '/node_proxy/data': support GET and POST requests. A GET will return the
  aggregated data for all nodes within the cluster. node-proxy will use a POST
  request to that endpoint to push its collected data.

Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>