omi: implement OMI `state` command

[facboy]

Currently omicliagent returns an error if the state command (without an argument) is sent. This commit fixes it to return the current OpenVPN state as required by the OMI spec.

[lstipakov]

real_time_status() doesn't really return connection state but status of real time notifications (on/off). I think the better approach would be to mimic openvpn2 behavior and return the latest connection state.

Here is my fix - it should end up in master eventually.