Fix traffic_ctl server drain(undo) jsonrpc api call.

[brbzull0]

It was wrongly calling admin_server_start_drain when it was supposed to call admin_server_stop_drain.

$ traffic_ctl server drain --undo -f rpc
--> {"id": "af5e7d34-8143-45be-98d0-448ad2c3cb01", "jsonrpc": "2.0", "method": "admin_server_stop_drain"}
<-- {"jsonrpc": "2.0", "result": "success", "id": "af5e7d34-8143-45be-98d0-448ad2c3cb01"}

[bneradt]

Pretty straightforward.

I'll let a non-Yahoo! approve it.

[bryancall]

@serrislew will review this

[serrislew]

Looks good to me.