set the environment for the supervisord process

If the supervisord.conf only has an extending rpcinterface, pay attention, **there is not any section [program:xxx]**, in this case, self.environment (the environment of ServerOptions) will never be assigned a value, which leads to two problems (given the example supervisord.conf as follows):

* the extending rpcinterface can not get the environment from os.environ
* the following function seems useless because **self.environment** never be assigned and **self.environment is None** 

        def process_environment(self):
          os.environ.update(self.environment or {}) 

This patch will fix this.

The example supervisord.conf and the background of this patch:

      [supervisord]
      logfile = /home/work/logtest/supervisord.log
      pidfile = /home/work/logtest/supervisord.pid
      subprocpidfile = /home/work/logtest/subprocess.pids
      environment=MYENV="true"

      [rpcinterface:supervisor]
      supervisor.rpcinterface_factory = supervisor.rpcinterface:make_main_rpcinterface

      [rpcinterface:myrpc]
      app_root = /home/work/apptest
      log_root = /home/work/logtest
      supervisor.rpcinterface_factory = myrpc.rpcinterface:myrpc_rpcinterface

**myrpc_rpcinterface** will get the environment MYENV in the supervisord section from os.environ.