[CI] Randomize RPCService's port and handle NoSuchProcess error properly

[withsmilo]

This PR contains two parts.

• Randomize RPCService's port in test_container_rpc.sh.
  We should avoid the port collisions between test cases as much as possible.
• Handle psutil.NoSuchProcess error properly.

 [unittest_rpc_container] Traceback (most recent call last):
 [unittest_rpc_container] File ../python/rpc_test_container.py, line 1, in <module>
 [unittest_rpc_container] import rpc
 [unittest_rpc_container] File /clipper/containers/python/rpc.py, line 20, in <module>
 [unittest_rpc_container] import clipper_admin.metrics as metrics
 [unittest_rpc_container] File /clipper/clipper_admin/clipper_admin/metrics/__init__.py, line 6, in <module>
 [unittest_rpc_container] if not server.redis_daemon_exist():
 [unittest_rpc_container] File /clipper/clipper_admin/clipper_admin/metrics/server.py, line 134, in redis_daemon_exist
 [unittest_rpc_container] process_names = [psutil.Process(pid).name() for pid in pids]
 [unittest_rpc_container] File /usr/local/lib/python2.7/dist-packages/psutil/__init__.py, line 338, in __init__
 [unittest_rpc_container] self._init(pid)
 [unittest_rpc_container] File /usr/local/lib/python2.7/dist-packages/psutil/__init__.py, line 378, in _init
 [unittest_rpc_container] raise NoSuchProcess(pid, None, msg)
 [unittest_rpc_container] psutil._exceptions.NoSuchProcess: psutil.NoSuchProcess no process found with pid 162

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[withsmilo]

Please review this PR. 😬

[rkooo567]

Looks all good to me!

[withsmilo]

@rkooo567 Thank you! 😊