rabbitmq-env.bat: Define $RABBITMQ_QUORUM_DIR

The variable was used in `rabbitmq-server.bat` & friends but not
defined, leading to a broken RabbitMQ on Windows.

Here is the crash logged early in RabbitMQ startup:

    2018-10-31 10:34:23.672 [error] <0.42.0> application_controller: unterminated string starting with "\\=/RABBITMQ_SERV": "\\=/RABBITMQ_SERVER_START_ARGSRABBITMQ_DIST_ARGSTARVAR
    2018-10-31 10:34:23.672 [error] <0.7.0>
    Error description:
        init:do_boot/3 line 819
        init:start_em/1 line 1111
        rabbit:start_it/1 line 460
        rabbit:broker_start/0 line 336
        rabbit:start_apps/2 line 518
        app_utils:load_applications/1 line 41
        app_utils:load_applications/2 line 110
    throw:{error,{bad_environment_value,"\"\\\\=/RABBITMQ_SERVER_START_ARGSRABBITMQ_DIST_ARGSTARVAR"}}

Author: dumbbell

scripts/rabbitmq-env.bat

@@ -262,6 +262,15 @@ if not exist "!RABBITMQ_MNESIA_DIR!" (
 )
 for /f "delims=" %%F in ("!RABBITMQ_MNESIA_DIR!") do set RABBITMQ_MNESIA_DIR=%%~sF
 
+REM [ "x" = "x$RABBITMQ_QUORUM_DIR" ] && RABBITMQ_QUORUM_DIR=${RABBITMQ_MNESIA_DIR}/quorum
+if "!RABBITMQ_QUORUM_DIR!"=="" (
+    set RABBITMQ_QUORUM_DIR=!RABBITMQ_MNESIA_DIR!\quorum
+)
+if not exist "!RABBITMQ_QUORUM_DIR!" (
+    mkdir "!RABBITMQ_QUORUM_DIR!"
+)
+for /f "delims=" %%F in ("!RABBITMQ_QUORUM_DIR!") do set RABBITMQ_QUORUM_DIR=%%~sF
+
 REM [ "x" = "x$RABBITMQ_PID_FILE" ] && RABBITMQ_PID_FILE=${PID_FILE}
 REM [ "x" = "x$RABBITMQ_PID_FILE" ] && RABBITMQ_PID_FILE=${RABBITMQ_MNESIA_DIR}.pid
 REM No Windows equivalent