I had the same problem in the last couple of days, but hadn't posted anything because I couldn't figure out _why_ I was having the problem.
I had the same symptons where I couldn't add nodes to a district. I _could_ `oo-mco ping`, but _could not_ make any mcollective rpc call. For example, `oo-mco rpc rpcutil inventory` would fail. I finally got it to work by changing `direct_addressing = 0` to `direct_addressing = 1` in `/opt/rh/ruby193/root/etc/mcollective/server.cfg` and restarting ruby193-mcollective on the nodes.
That parameter used to be set to 1 in OpenShift Enterprise 1 [oe1], but was changed to 0 in Enterprise 2 [oe2]. The mcollective documentation says that it _should_ be set to 1 [mc].